Always working workaround (Manual download)
Download directly from Apple:
Latest Release version
or
Latest Beta version
or
Any version you need. And all other downloadable contents
Then extract and move it where you like. This has so many benefits comparing to appstore update. (like the ability to resume download, not replacing the old one, not wasting hidden directories and etc.)
Note that you should use safari to download it.
Apple suggested workaround (Force App Store to redownload)
Since This is Known Issues
Xcode may fail to update from the Mac App Store after updating to macOS Catalina. (56061273)
Apple suggest this:
To trigger a new download you can delete the existing Xcode.app or temporarily change the file extension so it is no longer visible to the App Store.
