You'll need to:
1. Locate the lever or knob: Feel around the side of the seat cushion. It's usually a fairly substantial lever, not a small button.
2. Pull or turn the lever/knob: The action required to release the recline will depend on the exact design of your lever. It might be a pull lever that you lift to release the locking mechanism, allowing you to recline the seat back. Or it might be a rotating knob that you need to turn. Try carefully pulling or turning in both directions to find what works.
3. Recline the seat: Once released, carefully lean back on the seat to adjust its recline to your desired position.
If you can't find a lever or knob, the seat back may be fixed in position. This is less common but possible, especially if the car has been modified.