You can check out the FAQ posts for info on the save system and the Public Test Branch:

https://steamcommunity.com/app/892970/discussions/0/3383905288030494726/

https://steamcommunity.com/app/892970/discussions/0/3589961352692129408/

I copied both, "AppData\LocalLow\IronGate\Valheim" and "\steamapps\common\Valheim". Tried out the mistlands and had some fun.
Decided to go back and play with friends. Deleted the folders, told Steam I'm not playing Beta anymore and renamed the backup copies back. Managed to restore the character with Steam Cloud. Lost the world.

Until they implement a tool that converts your saves between versions, I'd advise to stay out of non-default versions.

Thanx for answer mate, but how do i easiest backup my saves?

Be careful, I almost lost my world . The steam cloud save was crucial. I would recommend waiting. It will be out soon and there are many vids on update to watch. Probably small updates in the first few weeks after it comes out .I am going to wait awhile then use a few mods because it will be a year or so until next update I am sure.Game should be stable and unchanged for along time. Remove mist mod will prolly show up soon, lol .

Then back up your saves before you try it.
Me and a couple of friends have played well over 100 hours on the new update, and other than a strange crash once, never lost anything except that tiny bit of time between saves.