1. Go to your server on our Control Panel.
2. Click settings in the server navigation bar.
3. Click world on the left menu.
4. The available worlds on your Minecraft server will be listed at the top.
5. Click the world you would like to play.
6. Click save and restart your server.

If you would like to create a new world you can follow the guide here.

Not using ServerFlex

1. Navigate to your server files.
2. Find the file called server.properties.
3. Open the file in an editor.
4. Find the line that contains "level-name".
5. Change this line to the folder name that contains your world.
6. Save and restart your server.