If you're wanting to delete your world, removing it is very easy. Just log into Multicraft select and stop your server then follow these steps:

1. Stop your server.
2. On the left hand side click "FTP File Access", and login.
3. Find the directory/folder that relates to your current world. 
4. Select it by ticking the box to the left of it.
5. Press the "Delete" button. (This will delete any files you have ticked/selected). 
6. Press "Submit" to confirm you wish to delete the selected files.
7. If you've deleted your current world, start up your server, and it will generate a new one.