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Use a utility knife to score around the old window casing and interior wall. Then, using a hammer and chisel, separate the casing from the reveal board.
You may have to remove some siding to get to the nailing fins that attach the old window to the house. Work your way up, prying up on the bottom of each piece of siding until you can get to the nails holding it in place. Take your time, work carefully, and try not to damage the siding as you remove it because you will be reinstalling it later. Some of the siding pieces may be very long and it may seem like a lot to remove for one window, but you'll save a lot of time and money later if you don't have to cut any new siding.
Place contact tape or heavy masking tape on the window glass to prevent breakage during removal.
Remove old caulk from around the window. Then, use a cat's paw to pry out the nails holding the old window in place, or you can use a reciprocating saw or mini-hacksaw to cut through the nails. Be careful not to damage the sheathing of your home.
Most windows can be removed in one piece, but depending on the type (e.g. double-hung, casement) and age of the window, you may need to remove parts of the window (e.g. bottom sash of a double-hung) before the rest of the window can be removed.
Regardless of what kind of window you are replacing the old window with, you'll want to make sure the rough opening is plumb, level and square. Measure all sides including corner-to-corner measurements in two directions. Once you have these measurements, you're ready to order your new window.
If you need to cover the opening until your new window arrives, make sure you tack your cover (e.g., plywood) far enough away from the opening so as not to put new holes in the sheathing where the new window flashing will be placed.