Current settings do not allow automatic updates. Please check version restrictions on moodle.org. I ended up manually updating because automatic download would not do Ver. 79 or 80 from 78.
I'm not sure what happened when you tried to update, but currently the HotPot plugin settings appear to allow updating from any previous version of the module (see screenshot).
If you continue to have problems, please let me know and we can elevate this issue to a higher authority.