Fairly simple to remove the old one and re-fit it.
I did mine recently when my rear window split due to the cold weather (and me being stupid and trying to lower the roof whilst it was -1c outside)
Haven't tried with a brand new one, I heard that getting them to mesh with the original zip is tricky, which is why I had someone unstitch the old window and sew in some new plastic. rather than risk having to DIY sew half of a new zip into the roof lining.
Anyway here goes:
From outside the car:
1) raise the window section of the roof, using the normal controls.
2) along the bar that screws into the car, there are screws/rivets to be taken out, theres about 8 of them, use an allen key to take these out.
From inside the car:
3) up inside the roof/window join there are 2 poppers on elastic, one each side, these need undoing
4) unzip the original, and you should be done.
Now try this in reverse with the new one..... Good Luck!