I scrape them clean with the edge of a sharp knife, then I use Scotch Brite, or one of the sponges with that green scotch-brite material....... to polish the heat sink clean.
CPU, would just be a clean cotton cloth.
ALSO: If it's running that warm, add a CASE FAN, or TWO.
** If the case doesn't have the ability to accept TWO fans, then buy a SLOT-FAN. They can be had for about $6 at newegg.com
If the computer is in a computer desk, one of those little computer ovens, then the minimum thing to do is take the back off, and let air circulate.