Insert the matching key into the mortice key hole. Insert the key all the way so that it cannot be pushed in the key hole any farther.
Turn the key counter-clockwise by one half to one full turn, depending on the model, to force the bolt to slide to the left from its locked position.
Listen closely to make sure the bolt retracts into the body of the lock from its fitting in the door frame. You'll hear a "click" when this occurs. If the bolt does not unlock, jiggle the key in the keyhole and try the previous step again.
Turn the door knob counter-clockwise, if applicable for that particular mortice lock fitting, or push down on the door handle for other models.