Lenovo ThinkCentre M55 - Manual (Page 20)

12. Make sure that the heat sink is cool enough to handle safely.
13. Release and pivot lever 1 upward. Remove the heat sink 2 from the
system board by carefully lifting it up and off the mounting bracket. Place the
heat sink on its side on a clean, flat surface so the thermal material on the
bottom of the heat sink does not touch the surface and become contaminated.
14. Release and pivot locking lever 2 securing the microprocessor 1 , then
pivot the microprocessor retainer 3 until it is open.
12 Hardware Replacement Guide