Most people will prefer to leave radiator installations to the professionals, but if you have good basic DIY skills, it's possible to replace an old unit with a replacement, using some basic plumbing techniques.
Firstly, turn off the radiator and let it cool. Take the protective cap off the lockshield valve and use a spanner to close this off. Check how many turns you use, as you'll need to open the valve again later using the same number of turns - this will keep the central heating system balanced.
Secondly, bleed the radiator, using a key to open up the valve in the radiator's top corner. You can then take off the old radiator, leaving both valves in place. Make sure you use a wrench to grip the valve assembly, so that it can't be pulled away from the feed pipe accidentally. Undo the nuts that hold the radiator to the valve, and close the bleed valve. Lift the radiator away from its brackets, and hold it over a bucket at an angle, so it can drain. You may need an extra pair of hands for this, as the radiator is likely to be heavy!
You can now install your new radiator, by wrapping a length of PTFE tape clockwise around the new model's thread. Lift the radiator into the brackets, and make sure the valves are sitting squarely onto the connections. Hand tighten each nut, and make sure that you don't accidentally cross thread them. You'll then need to tighten the nuts with a wrench - but don't over tighten it. Open the bleed valve at the top, and the flow valve at the bottom of the radiator to allow water to flow and push the air upwards. Close the bleed valve when water begins to leak from it.
You can then open the return valve, and allow the water to move through the system. Check for any leaks before you switch the heating on, and then check again!