You have to take the door apart. I have done this on my truck. I have never done it on a Jetta.
The first step is to take the door apart. Look for screws, obvious and hidden. Phillips, hex, and Torx are going to be your most commonly encountered ones. When you get the inside covering of the door off, you should be able to get hold of the window.
And beyond this, I cannot help you.
JayR