How to bleed a radiator.
Trapped air stops warm water circulating around your radiator. It's what makes the radiator cold at the top but warm at the bottom.
Start by turning the heating off and, most importantly, wait for the system to cool down - you won't then burn yourself if water escapes.
The bleed valve is located at the top of the radiator on either the left or right side and looks like a square screw or bolt. Use a radiator bleed key or flat head screwdriver. Turn the screw anti-clockwise about a quarter of a turn or until you hear a hissing noise: that's the air coming out. Wait until the air has escaped. When that happens, water will start seeping out. Then tighten the screw back up, but don't over-tighten. Your radiator should now work perfectly.