Lenovo ThinkCentre M55 - Manual (Page 26)

4. Make sure that the heat sink is cool enough to handle safely.
5. 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.
6. Release and pivot locking lever 2 securing the microprocessor 1 , then
pivot the microprocessor retainer 3 until it is open.
18 Hardware Replacement Guide