No longer admin on my plex home after accepting invite to friend's

[OttoKerner]

You need to leave his Plex Home, which you joined.
This will make you a regular “shared user”, instead of a Home member.

Someone took admin from me

Go to https://app.plex.tv/desktop#!/settings/users-sharing Hold the mouse over the line with your user name. There is a red X at the right end, which reads ‘Leave Home’.