I have watched a lot of videos on removing the pane and reglazing. It doesn't seem that hard. I'm stuck though on how to remove the sash. I have a video I made which goes over the entire window.
Pretty sure the sash is stuck within the frame.
However, there might be glazing stops that you can gently pry off with a stiff putty knife. (This is hard, btw. Every one I've ever met has been thin and easy to mangle.)
If you get the stops off successfully, you can order a new thermal unit from a specialty place. (While you're there, show them the stops to see if they have new ones.) Install is pretty straightforward, though you'll be happier if you have a couple of suction cup grabbers.