Rename your Valheim World
Just enter your new name, choose your .fwl file and hit the rename button!
More Info
The .fwl is a binary file containing your Valheim world metadata. If you want to or somehow must rename that file and use it (for example on your dedicated server) you run into problems. You will probably spawn in a new generated world, which looks like your old one, but without your progress like buildings, chests, dungeons, etc.. Even some commercial server providers don't get this renaming right, and you run into problems with some of them too.
To save you from all that hassle, use this site to quickly rename your Valheim world without corrupting your world files. Nothing of your .fwl is stored on this server.
After you get your new .fwl file, just rename your .db filename and copy it together with your new .fwl into your worlds directory.
Your world files are located at:
windows: C:\Users\%username%\AppData\LocalLow\IronGate\Valheim\worlds
or ..worlds_local
linux: ~/.config/unity3d/IronGate/Valheim/worlds
or ..worlds_local
Enjoy the awesome game and have fun playing Valheim!