1. Navigate to the %userprofile%\AppData\LocalLow\IronGate\Valheim\worlds folder on your computer.
2. Click on both the .db and .fwl file (no other files) and right-click, Send To, Compressed Zip folder.
3. In the newly generated .zip file, rename the .db and .fwl files to match your Server ID, while keeping the extensions.
(Your Server Id can be found in your control panel.)
https://www.survivalservers.com/wiki/index.php?title=How_to_Backup_Or_Upload_Save_Data