First of all, backup your world using the "Backup World" function in the gamepanel.
Unfortunately, due to the size of the file that needs editing, you won't be able to edit it using the File Manager.
You'll have to download the file via FTP. Accessing your files | File manager | FTP
The file you want to edit is named SANDBOX_0_0_0_.sbs and is located at Instance\Saves\<WORLD_NAME_HERE>
You'll need to transfer that file to your PC, open it using an editor such as Notepad++ then change the value for <SurfaceGravity>1</SurfaceGravity> to your desired value for any planets you want to change.
Save the file then upload it to your server.
Then delete the SANDBOX_0_0_0_.sbsB5 file and startup your server.
