First, make sure you have started your server at least once for the ".config" folder to appear. Then, stop your server, and follow these steps.
In the "File manager" tab on the server panel, browse to the ".config" folder and then "unity3d", followed by "IronGate", "Valheim" and "Worlds". (.config/unity3d/IronGate/Valheim/Worlds)
Note that your server must have the same world name as the world files you upload. The server world name can be changed in the "Startup" tab on the panel (the option is "World Name" and is different from the "Server Browser Name".) I. E. if your world name is "AmazingServer", your world files will need to be named "AmazingServer.db" and "AmazingServer.fwl".
Start your server, start playing!