How do I manage, back up, or replace my Valheim world?

Your active world is a matched pair of files in .config/unity3d/IronGate/Valheim/worlds_local - note that is a hidden dotfile path, not the server root, so you will not find it by clicking around from the top level. In there you will see: <worldname>.db (the world data) and <worldname>.fwl (metadata, including whether it's a custom seed). Both files share the name set in your Startup page world name variable; the server will not find your world if the files and the variable disagree.

To back up your world, use the Backups page rather than only relying on the raw files; scheduled backups protect you if a crash or bad shutdown corrupts a save mid-write. You can also download the .db/.fwl pair directly via File Manager for an off-panel copy.

To move an existing single-player or another host's world onto your server, stop the server, upload that world's .db and .fwl files to the same .config/unity3d/IronGate/Valheim/worlds_local folder via File Manager, rename them to match your Startup page world name exactly, and start the server. To start completely fresh instead, stop the server, delete or rename the existing .db/.fwl pair (back them up first if there is any chance you will want them back), and start; Valheim generates a new world on that next boot using the world name and any seed you configured.

A world seed can only be set at creation. If you want a specific seed, you need to set it before that world's files exist, either by wiping first or by choosing the seed on your very first start.