Should be straight forward. I've replaced our windows and have been putting off replacing the sliding glass door, partly because it's an oddball size and I need to order a custom one.
Pulling the trim off on both sides should show you where the frame is anchored to the studs, then it's just a matter of pulling the nails or sawzallin em through the gap.
Slide old frame out slide new one in, anchor in place and use a spray foam to seal the gap, trim it out and paint trim.
Those are the basics.
