Hi,
I had the same problem a few years ago, the solution I found was ....
If you look down the side of where the roof connections, there is a metal part that once pushed in will free the roof, you will need to have a partner on the other side to do the same, you'll just need to stand where the rear wheel is and look in while sticking your fingers through to the metal part, both your partner and you then push this part and at the same time with your other hand pull the back window out.
You shouldn't need to force anything!
I hope this helps, it should if done correctly 😉