You can change the name of a world you already have been playing on by following these steps:
- Log in and go to your dashboard here.
- Select the server you want to change and hit the Activate button to start your server.
- Once your server has started, go to the File Manager tab from the top menu.
- You will see all of the files for your server, including your World folders.
- Select the file for the world you want to change and right-click.
- Click on Rename, enter your new world name and click Apply.
(Be sure not to include any spaces or special characters in your name as it will cause issues if you need to download it) - Now you will want to set this new world as your Default to ensure you will join it each time. You can learn more about how to do that here!