There are usually two ways to migrate your project from an older version of swift into the latest version. First of all, you should download and install the latest Xcode either from AppStore or Apple Developer center. Secondly, you have an option of either from manual mode or let Xcode do it for you.

Change it from your project setting -> Swift Language Version, from the dropdown, select the latest version of Swift, like 4.2. Then compile/build your code, fix each warning/error one by one with some hint for Xcode like the following.

Moreover, XCode should also be able to migrate all your old version swift code into new version but somehow, we always have third party libraries or some other dependencies in the project. From my personal experience it never worked for me for a single time when projects are more complicated or with a huge code base. You may still have to go back to the manual mode again.

I've listed all migration issues I met when I was migrating from Swift 4 to 4.2 below. Please feel free to add anything I'm missing with your Pull Requests and for sure there would be a lot. With this document I hope it will help user with their migrations go smoothly and it would also be a good reference to when reviewing legacy code.

UIKit

#Swift4/UIKit

UITableViewCell

Swift 4	Swift 4.2
UITableViewCellStyle	UITableViewCell.CellStyle

UIEvent

Swift 4	Swift 4.2
UIEventSubtype	UIEvent.EventSubtype

UITableView

Swift 4	Swift 4.2
UITableViewScrollPosition	UITableView.ScrollPosition
UITableViewAutomaticDimension	UITableView.automaticDimension
UITableViewCellEditingStyle	UITableViewCell.EditingStyle
UITableViewRowAnimation	UITableView.RowAnimation
UITableViewStyle	UITableView.Style
UITableViewCellAccessoryType	UITableViewCell.AccessoryType

UIControl

Swift 4	Swift 4.2
UIControlEvents	UIControl.Event

UIWindow

Swift 4	Swift 4.2
UIWindowLevelAlert	UIWindow.Level.alert
UIKeyboardFrameEndUserInfoKey	UIResponder.keyboardFrameEndUserInfoKey
UIKeyboardFrameBeginUserInfoKey	UIResponder.keyboardFrameBeginUserInfoKey
UIKeyboardAnimationDurationUserInfoKey	UIResponder.keyboardAnimationDurationUserInfoKey
UIKeyboardAnimationCurveUserInfoKey	UIResponder.keyboardAnimationCurveUserInfoKey
UIKeyboardIsLocalUserInfoKey	UIResponder.keyboardIsLocalUserInfoKey
UIWindowDidBecomeVisible	UIWindow.didBecomeVisibleNotification
UIWindowDidBecomeHidden	UIWindow.didBecomeHiddenNotification
UIWindowDidBecomeKey	UIWindow.didBecomeKeyNotification
UIWindowDidResignKey	UIWindow.didResignKeyNotification
UIKeyboardWillShow	UIResponder.keyboardWillShowNotification
UIKeyboardDidShow	UIResponder.keyboardDidShowNotification
UIKeyboardWillHide	UIResponder.keyboardWillHideNotification
UIKeyboardDidHide	UIResponder.keyboardDidHideNotification

UIViewController

Swift 4	Swift 4.2
open func addChildViewController(_ childController: UIViewController)	open func addChild(_ childController: UIViewController)
open func willMove(toParentViewController parent: UIViewController?)	open func willMove(toParent parent: UIViewController?)
open func didMove(toParentViewController parent: UIViewController?)	open func didMove(toParent parent: UIViewController?)
open func removeFromParentViewController()	open func removeFromParent()

UIActivity

Swift 4	Swift 4.2
UIActivityType	UIActivity.ActivityType

UIActivityIndicatorView

Swift 4	Swift 4.2
activityIndicator.activityIndicatorViewStyle	activityIndicator.style

UIAlertController

Swift 4	Swift 4.2
UIAlertActionStyle	UIAlertAction.Style
UIAlertControllerStyle	UIAlertController.Style

UIPageViewController

Swift 4	Swift 4.2
UIPageViewControllerNavigationDirection	UIPageViewController.NavigationDirection
UIPageViewControllerSpineLocation	UIPageViewController.SpineLocation
UIPageViewControllerNavigationOrientation	UIPageViewController.NavigationOrientation
UIPageViewControllerTransitionStyle	UIPageViewController.TransitionStyle
UIPageViewControllerOptionsKey	UIPageViewController.OptionsKey

UINavigationController

Swift 4	Swift 4.2
UINavigationControllerOperation	UINavigationController.Operation

UIGestureRecognizer

Swift 4	Swift 4.2
UIGestureRecognizerStatePossible	UIGestureRecognizer.State.possible
UIGestureRecognizerStateBegan	UIGestureRecognizer.State.began
UIGestureRecognizerStateChanged	UIGestureRecognizer.State.changed
UIGestureRecognizerStateEnded	UIGestureRecognizer.State.ended
UIGestureRecognizerStateCancelled	UIGestureRecognizer.State.cancelled
UIGestureRecognizerStateFailed	UIGestureRecognizer.State.failed
UIGestureRecognizerStateRecognized	UIGestureRecognizer.State.recognized

NSLayoutFormat

Swift 4	Swift 4.2
NSLayoutFormatOptions	NSLayoutConstraint.FormatOptions

UIEdgeInsets

Swift 4	Swift 4.2
public func UIEdgeInsetsMake(_ top: CGFloat, _ left: CGFloat, _ bottom: CGFloat, _ right: CGFloat) -> UIEdgeInsets	UIEdgeInsets(top: CGFloat, left: CGFloat, bottom: CGFloat, right: CGFloat)
public func UIEdgeInsetsInsetRect(_ rect: CGRect, _ insets: UIEdgeInsets) -> CGRect	public func inset(by insets: UIEdgeInsets) -> CGRect

UIFontDescriptor

Swift 4	Swift 4.2
UIFontDescriptorSymbolicTraits	UIFontDescriptor.SymbolicTraits

UIImage

Swift 4	Swift 4.2
UIKIT_EXTERN NSData * __nullable UIImagePNGRepresentation(UIImage * __nonnull image);	public func pngData() -> Data?
NSData * __nullable UIImageJPEGRepresentation(UIImage * __nonnull image, CGFloat compressionQuality);	public func jpegData(compressionQuality: CGFloat) -> Data?

UIApplication

Swift 4	Swift 4.2
UIApplicationDidEnterBackground	UIApplication.didEnterBackgroundNotification
UIApplicationWillEnterForeground	UIApplication.willEnterForegroundNotification
UIApplicationDidFinishLaunching	UIApplication.didFinishLaunchingNotification
UIApplicationDidBecomeActive	UIApplication.didBecomeActiveNotification
UIApplicationWillResignActive	UIApplication.willResignActiveNotification
UIApplicationDidReceiveMemoryWarning	UIApplication.didReceiveMemoryWarningNotification
UIApplicationWillTerminate	UIApplication.willTerminateNotification
UIApplicationSignificantTimeChange	UIApplication.significantTimeChangeNotification
UIApplicationWillChangeStatusBarOrientation	UIApplication.willChangeStatusBarOrientationNotification
UIApplicationDidChangeStatusBarOrientation	UIApplication.didChangeStatusBarOrientationNotification
UIApplicationDidChangeStatusBarFrame	UIApplication.didChangeStatusBarFrameNotification
UIApplicationBackgroundRefreshStatusDidChange	UIApplication.backgroundRefreshStatusDidChangeNotification
UIApplicationProtectedDataWillBecomeUnavailable	UIApplication.protectedDataWillBecomeUnavailableNotification
UIApplicationProtectedDataDidBecomeAvailable	UIApplication.protectedDataDidBecomeAvailableNotification
UIApplicationUserDidTakeScreenshot	UIApplication.userDidTakeScreenshotNotification
UIApplicationOpenSettingsURLString	UIApplication.openSettingsURLString
UIApplicationLaunchOptionsKey	UIApplication.LaunchOptionsKey
UIInterfaceOrientationIsLandscape()	UIApplication.shared.statusBarOrientation.isLandscape

UIView

Swift 4	Swift 4.2
func bringSubview(toFront view: UIView)	func bringSubviewToFront(_ view: UIView)
UIViewAnimationOptions	UIView.AnimationOptions()

Swift 4	Swift 4.2
kCATransitionPush	CATransitionType.push
kCATransitionFromBottom	CATransitionSubtype.fromBottom

Foundation

NSAttributedString

Swift 4	Swift 4.2
NSAttributedStringKey	NSAttributedString.Key

QuartzCore

CAShapeLayer

Swift 4	Swift 4.2
kCALineCapRound	CAShapeLayerLineCap.round
kCALineCapButt	CAShapeLayerLineCap.butt
kCALineCapSquare	CAShapeLayerLineCap.square
kCALineJoinMiter	CAShapeLayerLineJoin.miter
kCALineJoinRound	CAShapeLayerLineJoin.round
kCALineJoinBevel	CAShapeLayerLineJoin.bevel
kCAFillRuleNonZero	CAShapeLayerFillRule.nonZero
kCAFillRuleEvenOdd	CAShapeLayerFillRule.evenOdd

Other resources:

https://swift.org/migration-guide-swift4.2/

https://github.com/apple/swift-evolution

https://developer.apple.com/videos/play/wwdc2018/202/

https://github.com/apple/swift