The tires need to be at ambient temperature when the pressure is checked and adjusted. This is referred to as having cold tires. Never release air from a warm tire. When it cools down, the internal air pressure drops, which can lead to underinflated or even entirely deflated tires.
The tires can warm up very quickly and should be considered warm if driven for more than approximately 1.5 km (1 mile). They often need about 3 hours of cooling down before reaching ambient temperature again.
Note
Tire pressure monitoring system warning
You should do any adjustments to the tire pressure when the tires are cold. However, if you need to adjust the pressure on warm tires, you may have to add up to 20-40 kPa (2.9-5.8 psi) to the recommended tire pressure. This will help to remove an active low tire pressure warning.
If you accidentally over-inflate the tire, press the metal pin in the middle of the valve to let out the excess air. Then check the pressure again.