It says: installed version 1.26.5 is higher, cannot downgrade, but ZapStore show a higher version available, 1.26.8, like in your screenshot.
Why? Is there no other solution than delete and reinstall?
The latest version of Keychat is now available for download on Zapstore.
Thanks nostr:npub10r8xl2njyepcw2zwv3a6dyufj4e4ajx86hz6v4ehu4gnpupxxp7stjt2p8 
Discussion
This might be a bug in Zap Store. You can temporarily download the latest version from GitHub.
I did update from Obtainium, let's see if it works at the next update in ZapStore.