Locate your Valheim world file from your PC, to do this, you'll need to navigate to your %AppData% folder, from there go into LocalLow > IronGate > Valheim > worlds your current directory should look like this: C:\Users\[YOURUSERNAME]\AppData\LocalLow\IronGate\Valheim\worlds
Once located, you will need to choose you worlds you would like to upload by selecting all the .db and .fwl files for the world and copying them to your clipboard.
Please note: The .old files are not necessary.
Navigate to your game panel by clicking "Login to Control Panel".
When you've arrived on your game panel, click "File Manager". Then, click on Worlds then, using the files you've acquired in Step 1, upload them to this directory.
Go back to the Control Panel homepage, and click "Commandline Manager", from here, click "Selected" to manage the existing command-line, or if you have a custom command-line, open the settings for that. Then, in the "World Name" input box, enter the exact name of the world you uploaded; or if you uploaded multiple worlds, enter the one you desire to load.
Make sure to press "Save", and then the server should restart automatically unless "Don't Restart" has been enabled.