To remove
The radiator turn both taps on the radiator to the off or -- Minus sign.
Then undo the nuts on the radiator have a bowl ready only undo a little so the water will run out slowly.
Then when water stops remove completely.
(Your water might be black near the end)
To reinstall
You should should buy some P.T.F.E tape only a couple of pounds.
Then wrap around the thread say twice with the tape on each thread.
Then put the radiator back and tighten the nuts not to much pressure to bend the pipes but enough to seal the joint.
You must then bleed the radiator when the system is on to get rid of the air block.
Unless its a new radiator as there is no need.
Hope this helps