Most of the portlights
I seen have to be removed from the inside. I used a heat gun to soften up the bedding material, (Not hot but warm to the touch - take your time) I used a fein multi master to go around the inside flange followed by a flat pry bar. It took about an hour to remove and fortunately they hadn't used 5200. The outside, at least on Vigah was more a decorative ring which the portlight flange seals
Come to think of it, you may just want to remove the outside ring and re-seal it. I had to remove the portlights to replace the bulkhead behind them.
The main question is, are they leaking and if so where and how long. There could be other problems underneath. Post a picture if you can.