There are two common failure modes of "A/C blowing warm":
If you have warm air blowing on both sides and cold air out of other vents your Heater Control valve needs replaced as it is stuck "open" and making the heater function. This is a common failure in our cars. THE HCV is mounted onto the driver wheel well under the air intake bits.
If you have warm air blowing on the passenger side and cold air on the driver side, you have low refrigerant in your A/C loop. Two steps are required here: 1) find out where the refrigerant went. This usually needs a professionally equipped shop to do, then 2) after the leak is resolved, recharge with proper level of refrigerant.