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Showcase doesn't update

That is the wrong screen. You need to click on List Add-ons in the left menu bar so that you can access the drop down menu to upgrade a specific addon.

Other method? If you are referring to those AUTO INSTALLERS, I do not recommend then for showcase as showcase is too large of an addon and for SOME (not all) server setups, core XF starts running the XML before the unzipping process and file uploads process is complete which will abort the upgrade.
No not the auto installer, I don't use those. I mean from the ACP home, I just click on the add on listed there and it opens the update process, which is how I usually do it. Never knew I could do it from the Listed add on page
That is a core xenforo function. Has nothing to do with addons themselves. Could be a browser issue. Maybe do a hard refresh and try clicking any of the addon links. Maybe try a different browser as well.
I did try clicking on several other add ons after failing with Showcase and the upgrade window opened for them and I tried several other times with Showcase but they failed too. Thus my post, but your way worked fine and now when I click on the Showcase RC1 it willl open an update window. Go figure, but all is fine now :)