Can NOT replace world on my purchased Realm

I attempted to replace current World on my purchased Realm, at least 3 times. The upload starts, then hangs up. My only option is to cancel. The world is small, 11 - 13 MB. Haunted Swamp Mansion. Realm management needs major improvement, at the Mojang end! Short video attached. Detailed version, with steps and rechecks shown, at YouTube: https://youtu.be/PZcoZWCgxYA