I am not sure about transferring a world to Realms as I don't have experience in Realms.
However, assuming you only need the world file, you can just download that off your server by selecting the world folder in your File Manager and pressing download.
Take note only users with a paid plan can download more than one file at once, so if you do not have a paid plan, the zipped world you download will be corrupted, and thus unusable on your end.