In order to refill a window air conditioner refrigerant, you will need to purchase a can of refrigerant and a gauge. You will also need to locate the low pressure service port on your air conditioner. The low pressure service port is usually located near the bottom of the unit. Once you have located the low pressure service port, attach the gauge to the port and open the valve. Slowly add refrigerant until the unit is full. Be sure not to overfill the unit, as this can damage it.