Generally it just involves removing a number of screws, then slowly wiggle the cooler a bit to get it loose, then remove fan cable.
Then you clean the GPU chip with either a kit
like this or similar (ask if you're not absolutely certain whether your choice of cleaning agent is fine). Check that thermal pads and such are still where they belong (either on the graphics card or the cooler, sometimes they wind up in a mix of both).
Reassemble by doing the initial unassembly in reverse. Fan cable in. Place the cooler on the graphics card. Screw it back together.