Remove the windscreen, undo the 2 screws each side holding the frame together (note the lengths - one is longer than the other), separate the frame and slide the glass out. This will give you access to the inside of the frame to fit a new stud.
Do it on a padded surface so as not to risk damaging the glass, and use plenty of soapy lube when reassembling.
When I did mine it was pretty manky inside and I ended up having to make some new brass blocks for the frame screws to go into but apart from that it was pretty straightforward. But then my car was over 20 years old at the time and had a pretty hard life. If your car is younger and better cared for then it shouldn't be a problem.
