If it is a plastic sticker, try to heat up the window from the other side of the sticker so it becomes somewhat lose. Then, you can gently peel it off and it should come right off.

However, it it is paper based sticker you will not have much luck since those are really fragile. If you want to clean your window completely (ruining the stickers) I would suggest using a screen cleaner foam and a razor. Soak the sticker with the foam and without applying too much pressure on the windshield (don't scratch it) use razor to peel it off.

PS: This was meant as a comment rather than an answer but I cannot comment.

If it is a plastic sticker, try to heat up the window from the other side of the sticker so it becomes somewhat loose, then you can gently peel it off and it should come right off.

However, if it is a paper based sticker you will not have much luck since those are really fragile. If you want to clean your window completely (ruining the stickers) I would suggest using a screen cleaner foam and a razor. Soak the sticker with the foam and without applying too much pressure on the windshield (don't scratch it) use a razor blade to peel it off.