To create a backup of your world save, first login to the hosting panel. The login details were emailed on initial sign up.
Navigate to your game service and select the "File Manager"
Open the Saved folder and right click the "Worlds" folder and compress
Right click the compressed file and download a copy of the world on your local PC
Please note: If you're getting an error compressing the worlds folder you may need to stop the server first. Make sure you run the "save" admin command in game before stopping the server to ensure all current progress is saved.