Try this:
There is a big phillips screw under the mirror (slide under the mirror and you can see it through the hole)
Remove it and tilt the mirror mechanism to come out from the top holding tabs.
The mirror is attached to a plastic base and it has a metal hook that goes through the mirror motor mechanism and locked in by a metal clip.
Pulls the metal clip to get the cracked mirror and the plastic base out. Now, simply insert your other mirror and the base and put the metal clip back on. You will have to force it a bit to get the mirror plastic base locked into the two motor wheels.
you are done!