Just to add a little bit to what Sky said...You will also likely need to remove the vertical aluminum strip that is on the edge of the glass. That strip is used to give you something to grip to pull the window open, and it's a weather strip, as well - it has a strip of fuzz in it to keep the rain/wind out. I’ve seen some boats that don’t have it, but it’s supposed to be there. If someone before you, as was the case for me, has siliconed or otherwise glued that strip to the glass, then you’re going to have to get a Dremel tool and carefully cut about a 1/4" off the top so that you can lift the glass up enough to get it out. If you try to loosen the silicone or glue with a putty knife, you will shatter the window - guess how I know.
Move the panel to the middle of the window frame to take it out. Once you lift the glass up, swing the bottom of the glass out of the track - this glass comes out to the OUTSIDE of the boat. Like Sky mentioned, a bottle jack will often be required so that you can spread the opening a little. I placed the bottle jack in the middle of the window opening, on the inside, on the window sill. I used pieces of 2x4 lumber to build up to the height of the window so that I could operate the jack to spread the opening just a tad. You’ll hear creaking noises, but don’t worry - the fiberglass isn’t cracking - it’s just the wood around the window making creaking noises. It only takes a tiny bit of leverage with the bottle jack to get the opening big enough to get the glass out - don't crank on it so much that you can see a distortion in the opening! (or you must might break/crack something).
There is enough room to slide the glass up into the top of the track if you get that “pull strip” off. That’s how the glass got put in there in the first place.
I, personally, would refrain from cutting the new glass to be smaller for easier re-installation. The fact that you need to spread the opening a tad keeps the glass secure and prevents it from getting jostled about in heavy seas where it can accidently get dislodged, tossed out, or broken.
If your window is stuck shut, well, you’re just going to have to muscle that for a while until you can get it un-stuck. I had one stuck and for five years, I could not get that thing to move no matter what I did. I picked and scraped out all the crap in the frame and still couldn’t move it. One day, something had fallen against the closed door to that room (the bow stateroom; would be the galley window on a 53MY), and my elderly, blind cat was stuck in that room with no way for me to get to him. I somehow came up with the strength like never before and got that window open from the outside. Must have been a rush of adrenaline knowing my little boy kitty was locked in that room. I sure didn’t want to have to break that window.