How to pause Xcode update?
I faced the same issue and what I did was the following:


Go to Finder -> Applications
Movde Xcode.app to trash. (This will alert you if you want to cancel update).
Agree to cancel update from alert.
Go to Trash -> Xcode.app, right click and click on "Put Back"
Re-start using Xcode and continue with your work.


If you do not wish to hamper your work while you wait for XCode to update and you are in a slow internet connection, go to https://developer.apple.com/download/more/ and download latest version of xcode dmg (close to 6 GB) after you login in your apple developer account.