i had to replace mine last month. i don't think there any tricks to taking it off. if you need to replace the whole side mirror then you gotta take off the door panal and then from there you can take off the mirror. take the plastic cover off, then theres 3 nuts to take off and unplug the power and it should come off.
The mirror glass can be removed without taking off the mirror. You just pull it straight out and it comes off it's pivot mount. This applies to non heated mirrors. I assume the heated mirrors have a wire attached to them. I partially pulled mine out when I wanted to replace the cracked frame but I assume if you pull the mirror off it's mounting( I believe two mounts) it should come free.
If you're just replacing the mirrors, use a flat head screw driver wrapped with a towel, and pop the mirror off as gently as possible. Carefull though because there as spring and and a c-clip/pin that you'll need for the install.