That's harder. I need to replace one of those myself. You need to remove the window frame from the door, and then the vent glass. There are two copper rivets you need to drill out, and then pound in two new ones. I've read that you want to make sure you buy the real Porsche window stop, not aftermarket. Pelican has them, but I can't seem to find the rivets.
Check out this thread:
http://forums.pelicanparts.com/porsche-911-technical-forum/566914-question-asked-but-never-answered-how-do-i-install-new-targa-window-stops.html
Then you have to be careful to set the window travel. If it goes too high, it will tear through the new stop.
Mark