On most GPU's you can remove Heat sink from the card itself (be sure to dissipate any static electricity first). To do this you will need to remove the card from its slot and get a screwdriver. If your heat sink (the casing on the GPU) does not wrap around both side of the card, removing it will be simple. If you flip the card over to the side with no covering on it (should look flat with a bunch of lines on it) you should see 4 screws (there may be more). These screws are what hold the heat sink onto the GPU. If you unscrew each of them, making sure to do this in a diagonal pattern so as not to damage the card, the heat sink should come right off. You should then be able to get to any clumps of dust. Make sure to apply more thermal paste to the GPU before putting the heat sink back on, and you should be good to go.