If the leader of your tribe has gone inactive, feel free to send in a support ticket about it and we will take care of the issue from there.
If you are hoping there is an in-game function to overthrow a leader... There isn't.
The best you can hope to do is gain support from your fellow tribe members and either A) Work your way up the chain of command and usurp his position by popular demand or B) leave the tribe and start your own with the players who support you as leader.