If your radiator is cold at the top & hot at the bottom it needs bleeding to remove air that has built up.
Turn off your central heating & let the system cool down to avoid burns.
Locate the bleed valve - it will be near the top of the radiator on one side.
Place a tub underneath to catch drips & hold a towel below the valve.
Using a radiator key, turn the valve anticlockwise until you hear a hissing sound - that’s the air being released.
Leave the valve open until water starts to come out in a steady flow, then close the valve again.
On some radiator models, the bleed valve may be around the back.
On a heated towel rail, the bleed valve could be on the top.
Or on some heated towel rails, the valve is on one side.
Depending on your heating system, you may need to top up the pressure after bleeding - check the manual to confirm.
Turn the heating back on & check for cold spots - repeat if necessary on any radiators that aren’t warm at the top.
If you don’t have a radiator key, modern valves with a groove in them can be opened with a screwdriver.