You need to peel back (from the mirror side) the rubber gasket in one spot. Pull it back in 1 place and put a butter knife (or similar) behind the exposed mirror. Pull the gasket back and slide the knife as you go. Be sure not to dig into the gasket with the knife. You want to slide the knife along and cut thru the glue that GM used. Start the knife on the longer (top) side. You can pull the mirror out, once the entire top and a little bit of each side is peeled behind the mirror.
It is pretty much as Rick describes it, with the possible exception of the "glue". Some mirrors are stuck as if glued, and others seem to just pop right out. Not sure why they are not consistent. When procuring a new glass at a salvage yard, I usually just cut the rim off the rubber part and make it a bit easier to remove the glass. One tip is to put your mirror in the sun to warm up the rubber part so it is easier to work with.
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