These 2 strips of wood create the track that the lower window sash slides in. Start by using a utility knife to cut through any paint that binds the window stop to the window casing or frame. Then gently shove a stiff putty knife between the stop and frame to gently pry it off. Try not to damage the wood. Slide the lower window to the top of the frame.
Many older windows have a metal track that holds the sash in place.
Carefully remove any small nails or screws that hold the metal in place. Once the metal tracks are loose on both sides, the lower window should freely swing away from the frame. Carefully unhook the sash cords that are still attached and allow them to pull up to the pulleys. There is a thin strip of wood that separates the 2 window sashes. Carefully pry out the strips on both sides.