Local files can be found at:
Windows: C:\Users\USERNAME\AppData\LocalLow\IronGate\Valheim
Linux: .config/unity3d/IronGate/Valheim

Steam Cloud save folder:
Windows: Steam\userdata\NUMERICSTEAMID\892970\remote
Linux: .steam/steam/userdata/NUMERICSTEAMID/892970/remote

If you click manage every so often you can move world file to local and then back to Steam cloud in two steps... thereby making a local backup.

also this link should point to your cloud saves GL !!

https://store.steampowered.com/account/remotestorage