I fitted a new one to a replacement windscreen I had a while ago. They come with a gel pad on the front which sticks to the windscreen and makes a "bubblefree" bond. The rain sensor is round and only about 40mm across and fits into the mounting on the windscreen. It's fixed to the mount with a round metal clip. If you take off your mirror, first by squeezing together the cable cover and pulling it upwards away from the mirror them turning the mirror anti-clockwise so it unclips, you'll see the back of the rain sensor and it's clip.
If it was me, I'd let the windscreen company do it. They do it all the time so should have come across it before. I've never taken a sensor off so I wouldn't know if it's stuck fast. My main worry would be breaking it when you're pulling it off.
