Dell OptiPlex 990 (Early 2011) - Manual (Page 28)

7. Loosen the captive screws in the order of 1,2,3 and 4.
8. Lift the heat sink assembly upwards, and remove it from the computer. Lay
the assembly with the fan facing downwards, and with the thermal grease
facing upwards.
9. Press the release lever down and then move it outward to release it from
the retention hook that secures it.
10. Lift the processor cover.
28