Turn off the heating system and let the radiator cool completely
Locate the radiator bleed valve at the top corner of the radiator
Place a cloth or small container under the valve
Insert a radiator key or flat screwdriver into the bleed valve
Turn the valve slowly counterclockwise until you hear air hissing out
Wait until water starts to come out steadily
Turn the valve clockwise to close it securely
Wipe away any spilled water
Check the boiler pressure and top it up if needed
Turn the heating system back on and check that the radiator heats evenly
Repeat for any other radiators that still have trapped air