Assuming you have a standard up/down window button:
Use a flat head screwdriver to pry the old button out of the panel. You may need to wiggle it back and forth until it pops out. Be careful not to damage the surrounding area.
Take the new button and insert it into the hole in the panel. Again, you may need to wiggle it around until it's securely in place.
Now reconnect your battery and test out the new button!