At the very top of the screen, on the right side, is a settings option. Click that and then go to "Edit Character".

Then when your list of characters appears press the "Star" on the picture of the person you want to be your main and it'll go gold. Then just hit "Save Changes" at the bottom.

Hope this helps!

Edit: You can also click on the background of each character to toggle it brown (on) and black (off) as to whether it appears on your profile or not which you may/may not want to do for any non primary characters.