How to remove side vinyl from the window

I live in a cold place. I already have double sided window but unfortunately, they are massive and basically make up the whole living room, thus making the living room very cold. I bought more double side window, each with the same size.

Anyways, I was about to install the double sided window and found out very quick that removing these plastic siding thingy are very hard:

I tried prying it, but it is so hard to get it in there. I tried knife, it works, but it bends, and I’m afraid that if I pry it too hard, I"ll crack the window. I can’t find anything in the internet about this. I don’t even know the name of that siding thingy. Just "vinyl siding"? In any case, that didn’t yield any google results. Any help would bey very much appreciated.