Ed:
You will need to remove the whole window assembly, then take out the screws that hold the mullions in place, and the strap that holds the main frame together, pull the frame apart enough to pop out the window needing repair.
You can tell whether any of your windows have been done before: The original seal (if SE-GI) is black and has a wavy top surface, bumps every 3/8 inch or so. The replacement will be different. If a Root kit, it is grey and smooth, actually looks slightly smaller.
The job isn't difficult, but you will need a helper to get the assembly down to the ground, and to put it back in place. I got a pair of suction handles at Harbor Freight ($15. total) and had my wife hold onto the one on the inside after I loosened the window, then from the outside, I got most of the weight of the assembly and we lowered it together onto the top of my 5 ft ladder. Easy after that.