How do I remove this window sash to replace the glass pane?
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.