In your Steam library right-click Valheim, select 'Properties' then 'Betas' and set it to none. Wait while the game does a small update.
You can find your save games in users/yourname/AppData/LocalLow/IronGate. (probably on the C: drive)
There should be a folder for characters and a folder for worlds. Copy your backups from before you entered the Beta version so that they replace the current saves.
You should then be back to where you were when you did the backup before starting the Beta test.
Hope this helps.