Carefully remove the trim from the existing window, inside and out. Set it aside; you can use it later if you are installing the same size window or you can cut it to fit if the new window is smaller.Our old windows were wood-framed built-ins. We used a reciprocating saw to cut through the nails that held it to the opening. We were able to remove the window completely intact in the hopes of recycling it at our local salvaged-goods store.Tip: Get a helper to hold the window and keep it from falling out of the opening as you remove the fasteners that hold it in place.

Windows