How to clean it

A vacuum is an option as is canned compressed air, a soft brush can also be helpful. Be careful not to damage or bend the fins.

As it happens, I have this same card in an old PC and have cleaned it several times. If you elect to use a vacuum, use the extension hose and just put your fingers over the end, then allow the suction to reach between your fingers to the radiator without letting the metal touch it so you don't bend the fins.

That should work well enough.

In the unlikely event that it is VERY dirty say outdoor use or smoke residue you might consider actually washing it with warm water, Yes, I can hear everyone saying that's insane, but in actual fact. the final stage of production is washing for computer circuit boards and will often take care of pernicious overheating, the only caveats are not to apply any power until it is ABSOLUTELY dry and of course, not wetting things that can't tolerate water such as paper stuck on labels that may contain vital information (or not).

Good luck!

Hmm... not to offend but this assumes you have removed the GPU from the computer first (LOL)