Originally posted by Nefrarya:

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.

It does not need to download the game again. For me it just downloaded a 35-ish MB "patch" to revert the game back to the previous version.

Some people are reporting that starting up the game after reverting back to a previous version is giving a glitchy, flashing screen. Those people have also reported that it seems to be a case of the game reverting to default settings when "unpatching" and using something like the GeForce Experience to fix the graphics settings and allow it to play.