To switch branches in your Steam Library, do the following (for the players, not the server):

Right click on the Valheim entry in your Steam Library, click Properties.
In the left column, choose Betas. Select "default_old" from the dropdown menu.
Password should not be needed.

Your Steam client will then go back to the old version (not sure if it needs to download the game again to do that). As soon as the server has updated, you can do the same thing again and choose "Non" in the beta dropdown menu. After that, you should be able to use the new features on your server worlds.

Edit: Don't know if this works for singleplayer games with mods.