First of all you must measure the windows correctly. If in brick, measure to the inside of the brick.if in wood or vinyl siding measure to the inside of the interior of window to get a more accurate fit.Purchase windows at a local provider,if in stock or order if necessary.
To remove the existing window.Cut the caulk around the window with a razor knife. If flange nailed to house . Remove all of the glass from the window frame and Pry window frame from wall using a pry bar and hammer..Cut in the middle of window frame with a reciprocating saw.(SawZall).Then pry window out, the rest of the way.If no flange remove screws hidden behind plastic strip and remove window. Clean opening out thoroughly with a shop vacuum.
Plumb and level the window on the inside of opening.Screw each corner until you achieve plumb and level. Use shims if necessary.Window should fit pretty snug. check to see if the window operates properly.Stuff insulation around the window to fill any gaps.Then silicone the outside around the brick or siding, anything that isn’t paintable.If the surface is paintable use painters caulk or paintable silicone.So that the outside window trim can be painted
Inside caulk window around drywall .Silicone window to sill if sill is not paintable,or painted.If excessive gap, add a quarter round trim or drywall.always wear eye protection and gloves when working with glass.