How to change your back brakes

i cant put the brake pad on becouse the round thing wont push back

There are a lot of "round things" on a wheel. If it's rusted, it may help to spray some WD-40 on the round thing, let it sit for about 30 minutes then gently strike it with a rubber mallet.

Hi.

No doubt the round thing is the piston in the brake unit. To push it back you need to open the brake fluid bleed nipple to let some brake fluid out so as to let the piston move back. Than afterwards you need to bleed the brake fluid system again adding more brake fluid under the hood.

Good luck, D.