okay, when you raise lvl 60, 66, 72 or 78. You can change your main.
You must to challenge her/him (other mains) to get her/ him as you main.
When you got him/her and you change intro him/her.
But when you change intro a other main. You can't not directly change back.
You must to wait 24 hours to change back and if you use ingots you can change anytime you want :P

Do you understand it now?