need help
 

alright so my back brake was rubbing on my older pro tour (garbage bike)
but im trying to upgrade it. so far i have it single speeded.
and i stripped some stuff off. like the front brakes, friction shifts obviously,
reflectors, and even took off the tape to put grips on my drops. it looks kinda dumb.
but i think it looks legit.
but anyways, i was taking off the bolt on the back of the brake, and ended up stripping it to nothing.
i have no idea what to do, and its the only way to take off the brake so i can fix it.
any sugestions on how to fix this bolt?

btw this is my first post