Dell Dimension 8400 - Manual (Page 28)

b. On the retention base locate the tab opposite the power supply. Press on the retention base tab until the heat sink pops up slightly.
c. Press out on the second retention tab while lifting the heat sink up and out of the retention base.
3. Push down and out on the socket release lever.
4. Open the processor cover.
NOTICE: Lay the heat sink down on its side.Do not touch or remove the thermal grease on the bottom of the heat sink.
NOTICE: If you are installing a processor upgrade kit from Dell, discard the original heat sink. If you are not installing a processor upgrade kit from Dell,
reuse the original heat sink and blower when you install your new processor.