To bleed a radiator look at the top of the side of it. It'll have 2 nut type things that are a couple of inches in diameter, 1 will be flat, 1 will have another small nut in the centre of it. Go to the 1 with the nut in & use either a radiator key or screwdriver (if it has the slot as modern 1s do) to slowly turn it until air hisses out. Have an old cloth/bowl to hand to catch the water that spills out & then give it a while to heat up.
Depending on what type of boiler you have, you might need to top up the pressure again afterwards (if you have a combination boiler you'll need to) which is really simple on most boilers. It's usually a case of turning 1 or a couple of little tap handles until it reaches the right level on the gauge.
If bleeding doesn't work turn all the other radiators off & leave the dodgy 1 on on it's own for about half an hour. That sometimes forces things around & sorts it out.
My radiators are cold at the bottom? Lol
Sounds like the sludge in your system could be building up meaning you could need a flush of some kind. Depending on the age of your system, the best thing is a Power Flush (also known as a Jet Flush) which involves hooking up a machine to your system & blasting a chemical around it to remove some of the [Poor language removed]. They're expensive due to the amount of time they take (depends on how bad the build-up is but they take at least a few hours) & the price of the machines & no places that I know of guarantee they'll work but the vast, vast majority sort the problem out.
I have a habit of forgetting about threads after I've posted in them so if you need any more help & I don't respond PM me a reminder