Express showed an update for the ANT/BT firmware and seemingly installed it. However, the watch didn't show any update ready to install - even after restarting the watch. Also, Express didn't show the update as available anymore since it had copied the file and was thinking it's going to be fine.
I've then used Android File Transfer to remove the file and upload it again as GUP3260.GCD (all upper case) and after unplugging, the watch now showed the "Do you want to install update?" screen as expected.
Did anybody else notice such behaviour?