How to Upload a Custom World to Your Minecraft Server

1. Find the world

For a singleplayer world, open your game's saves folder (in the Minecraft Launcher: Installations → folder icon → saves). Each folder there is a world. A downloaded map is usually a .zip with a world folder inside.

The world folder is the one that contains level.dat.

2. Zip it

Zip the world folder on your computer. One zip uploads much faster than thousands of small files.

3. Upload it

1. In Console, click Stop.
2. Go to Backups and create a backup of the current world.
3. In Files, delete or rename the current world folder (and world_nether and world_the_end on Paper).
4. Upload your zip, and use Extract here on it.
5. Rename the extracted folder to world, or set level-name in server.properties to its name.
6. Make sure level.dat is directly inside it, not one folder deeper.
7. Click Start.

Paper and the Nether and End

Singleplayer keeps the Nether and End inside the world folder (DIM-1, DIM1). Paper keeps them in separate folders. Paper moves them across automatically on the first start; nothing to do.

Versions

Run the server on the same or a newer Minecraft version than the world was last played on. Opening a world on an older version can break it.