Postgres 15 is out – advice on how to upgrade would be appreciated!
From the docs, https://fly.io/docs/reference/postgres-on-nomad/#upgrading-the-postgres-app the update command would be:
flyctl image update -a <postgres-app-name>
That would get the latest version of their postgres app. However I believe the latest version of the app uses 14, not 15 
. So … if you didn’t want to wait for Fly to update their app, I guess you would need to fork your own version of their app. Which would let you do whatever you want with it. They suggest that approach in their readme:
there’s more info here: Postgres 15 · Issue #88 · fly-apps/postgres-ha · GitHub