i can't update Xcode

Try restarting your computer to see if Xcode has installed. This will also refresh the app store, so try downloading it again.

If all else fails:

You should uninstall all current versions of Xcode on your Mac. Do this by dragging each application to the Trash can. Delete your trash.

After, try downloading Xcode again and everything should go through.

You aren't aloud to download old versions of Xcode from the App Store.