Open your steam library
Right click on Valheim
Select properties
Click on the Betas menu
Click the drop down menu from the top box
Select default_old - previous stable version

Get in contact with your friend and let him know he needs to update. If you have been playing on a server the server admin needs to update to the newest version. If you aren't the server admin/owner you have to wait until they do so or until the server auto updates.

Some companies that provide server hosting are slow with the updates. If you complain they usually give you free time or a discount.

The above has happened to me a few times. It usually gets updated within 24-48 hours.