Open your township and press the town hall building.
◾ Do not assign a stranger to be a leader's assistant!
◾ If you (the leader) leave your township, it will be absolutely destroyed!
◾ Several or even all the township players may be assigned to help you out.
◾ Leader's assistant can neither assign this role to other players, nor remove or change the status of other leader's assistants!
◾ Only VIP 1+ players can have the leader's role (exception, see here)
◾ If you had already transferred the leadership to another player, but then changed your mind, the new leader has to contact the support team and transfer the township back to you.