You need a trim removal tool. I recommend applying a soft tape around the perimeter of the mirror before starting. Use the tool to start prying the mirror off the back, working your way all around. Keep in mind that you will have a plug to disconnect if you have the mirror defrosters. From there, you'll remove the plastic backing that the mirror attached to, and from there, you will be able to:
a) access a clip that you can use to remove the bulb cover, or
b) unscrew the bulb connector and remove the connector from the housing.
Then, you can switch it out to an LED.