Dell OptiPlex 755 User Manual page 265

15 Lower the heat sink onto the processor at a 45-degree angle so that one end of the heat sink fits
securely under the tab opposite the release tab on the retention base.
16 Re-assemble the shroud.
17 Replace the computer cover (see "Replacing the Computer Cover" on page 317).
18 Replace the cable cover, if used (see "Cable Cover (Optional)" on page 237).
1
heat sink
2
retention base
3 release lever
1
2
3
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Processor