I believe you cannot at the moment as it may still be bugged. To do so though, /changeleader (old leader username) (new leader username) The leader at the moment has to do so
The only working way is for the current leader to remove everyone from coop except for the person that they want to make the new leader. Then, current leader does /coop leave. The remaining coop will automatically become the new leader, and can reinvite everyone back.
Adding to this, if you cant kick anyone from your coop for example. And you left the coop, the oldest member on your island aka the first player who you invited to your island becomes the leader.
Closing since your resolution has been stated. If this is still an issue please feel free to create another thread or PM a moderator.
