Originally posted by umop-apisdn:

In the games list: right-click Valheim, choose "Properties".
Select "Betas" in the menu on the left.
Select "default_old" in the drop-down on the top-right.

The problem with this solution is that "default_old" also gets updated. So, if another little patch comes out tomorrow, your game will be broken again with no fix. What they should do is have several versions available in the betas tab.