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