Before you proceed, its important that you have a backup of your world file(s) just in case!
 
Modifying your world modifiers can be done over at: https://valhost.net/worldmodifiers
 
The site expects you to upload a .fwl file and then you can make the modifications you want.
 
The .fwl file of your server is found in .config/unity3d/IronGate/Valheim/worlds_local on your server in the File Manager tab.
 
Select to download it, and then upload it to the site above. Then, once you have made the modifications you want, you download it from the site and upload it back onto your server. Make sure the uploaded .fwl has the same name as your previous one.
 
Then, restart/start your server.