AUTO_UPDATE=1 (default) makes the server check Steam on every start and install a new version before launching. After Iron Gate releases a patch, players on the new version cannot join until you restart the server from the panel. That is all it takes.
Version mismatch
"Incompatible version" on join means the client and server differ. Restart the server (installs the update). If a player is on the old version, they need to update Steam.
Public test branch
To run the public-test build set SRCDS_BETAID to public-test and SRCDS_BETAPASS to yesimadebackups (Iron Gate's password for the branch), then Settings → Reinstall. Players must opt into the same branch. Clear both to return to the stable version.
Restart schedule
Valheim servers grow in memory over days. A nightly restart keeps them snappy: Schedules → cron 0 5 * * * → Power action: restart. The game saves every 30 minutes and on shutdown, so a scheduled restart loses nothing.