Good luck
Have done his. First, go to Youtube, great videos. To remove windows, you will have to knock out the glass, then just pry the old windows out. Tacked to the frame by a light metal strip around the outside of the window. To buy new windows, measure the inside and outside dimensions. Can order at Lowes, Home Depot, etc. You want a window or windows 1/4 inch smaller than the outside opening t allow installation. There is a ‘mullion’ that connects two windows together. Installed by 3 screws along the left and right. You can also secure to top and bottoms. You may need to get furring strips to provide something to attach to on the inside of the window frame, depending on how it is constructed.
If you are handy, you can do this. You will need another person to help put the window in place.
Just give yourself a couple of hours. WindowWorld did one of my big ones, took 30 minutes.