Most replacement mirrors are supposed to have a sticky pad on the back of the mirror, you peel the wax paper off the sticky pad then push/stick the mirror over the broken mirror.
If it doesnt have a sticky back then there maybe a supply problem (I've worked in garages on parts and have seen this before) If the mirror came with clips then you will have to follow the instructions if came with any, not personally seen mirrors with clips but I assume you would have to remove the old broken mirror from its housing (The bit that surrounds the actual mirror, not the plastic outer housing).