You can set a Leader of an event from both the administration area and from the public page of an event. These screens show how to do it from the administration area.
Then click ‘Events & Sessions’. Select the event you wish to edit.
Now scroll down to the People area, and choose the person you wish to make a leader.
Click on that user to see the user edit screen.
Click ‘Set as Leader’ to make that person a leader.
The selected user will appear with a purple border.