With both sashes open, remove the window screen brush away any debris, then wipe down the exterior glass with glass cleaner or a 50/50 solution of water and white vinegar. When finished, replace the top sash first by tilting it up and pushing until it snaps into the frame. Then, fully raise it to close the sash. Repeat with the bottom sash.
NOTE: If the window sashes do not close properly, they may need to be squared-up in the frame. Tilt the sash back in and ensure the bottom corners of each sash are level in the frame, then try raising the sashes into place again.