I had this issue while on xcode 12.5 and trying to upgrade to xcode 13.4, I didn't see an update button but instead an open button. After several minutes of researching for solution I decided to go through xcode page on appstore and discovered that I need macOS 12.3 to use the xcode, (i was running on MacOS 11.x). I updated my macOS and completed the installation, and voila! I can now see the update button. I hope this helps someone.
hilory
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