If the air conditioner is not cooling properly it will be low on refrigerant for two reasons. It was either undercharged during installation or the last tune-up or the air conditioner is leaking refrigerant.
If the air conditioner has a leak just adding refrigerant is not a fix. This is the time to call your local air conditioning repair service. You need to repair the leak in your a/c, test the repaired leak and then recharge the system with the correct amount of refrigerant. This is definitely a job for an air conditioning technician because air conditioners work best when the amount of refrigerant matches exactly to the manufacturer’s specifications and fixing the leak isn’t trivial. If the refrigerant is undercharged or overcharged the air conditioner’s performance will be off and can cause issue with the a/c down the road.