Open the bottom sash and remove the sill stop from the sill by backing out the holding screws with a Philips screwdriver in a counterclockwise direction. Pry up the stop with a flat blade screwdriver, if necessary and place it aside. Skip this step if your window is not equipped with a sill stop.
Lower the top sash completely. Remove the two screws in the left hand jamb liner (the track that both sashes slide on) with a Philips screwdriver in a counterclockwise direction. Now raise both sashes all the way up to access the jamb liner screws at the bottom. Remove the screws with a Philips screwdriver. Place all of the screws aside.
Hold the left hand jamb liner against the side of the window frame near the bottom of the sashes with one hand while sliding the liner out from the bottom with the other hand. Place the liner aside.
Close the lower sash and pull the left hand side inwards and away from the left hand window frame. Pull the left hand sash cord down (which is still attached to the sash) with one hand and hold it in place. Now lift the sash cord tab from the side of the sash with the other hand. Slowly let the sash cord retract back into the window frame, being careful as the cord is under spring pressure. Pull the right side of the sash away from the window frame and remove the right hand sash cord in the same fashion.
Push the upper sash completely down if it needs to be removed. Remove the sash cords in the same fashion as the lower sash. Skip this step if you do not need to replace the upper sash.
Install the upper sash first, if necessary. Place the groove in the right hand side of the sash into the sash track on the right hand window frame. Pull the sash tabs down from each side of the window frame and insert them into the slots on each side of the sash. Push the left hand side of the sash into the frame and raise it to the top of the frame. Repeat this step for both sashes.
Push the left hand sash liner into place between the left hand window frame and the sashes and slide it upwards until it reaches the top of the window frame. Replace the lower sash liner screws. Lower both sashes and replace the upper sash liner screws. Close the window.
Replace the sill stop and tighten the holding screws. Skip this step if your window does not have a sill stop.