When you are in the costume box menu, there are 2 buttons. one on the left, one on the right. When pressed, they switch between different costumes! The firefighter has no use yet, but the other 2 do!
I only see one button, there was no other ones2 days ago(+1)
The little blocks on the left and the right side of the screen are the ones you need. those are the buttons. i know they arent the best sprite, but i didnt have time to change them :)