Open the window panel by unlatching its latches -- often located on the top middle part of the window -- and pulling forward. The window will fall toward you. Rest it -- it is still attached but laying at a diagonal -- on the windowsill.
Squirt household disinfectant with bleach on the inside of the window. Wipe away the mold and dirt with a sponge. Wipe down with a damp sponge.
Spray glass cleaner on the inside glass. Wipe off with a clean, dry cloth.
Push the window back into position and latch it.