I'm not sure what kind of thermal transfer material is already present; if it's sort of a thin paste, then alcohol, tissue, and a Q-tip will do the trick fine. If it's sort of gummy, and more solid, like bubble gum, then a knife could be used to scrape it off. Just be careful not to make any indentations in the heatsink, as they will impact thermal transfer. Alcohol can then be used to get rid of any remnants of the gummy thermal material.