Lenovo ThinkCentre M55 - Manual (Page 22)

16. On the new system board, release and pivot the locking lever, then pivot the
microprocessor retainer until it is open.
Note: There will be a black plastic cover on the retainer to protect the
microprocessor socket. Remove the black plastic cover and place it on the
microprocessor socket of the defective system board.
17. Position the microprocessor so that the notches on the microprocessor are
aligned with the tabs in the microprocessor socket.
Important
To avoid damaging the microprocessor contacts, do not tilt the
microprocessor when installing it into the socket.
18. Lower the microprocessor straight down into the microprocessor socket.
14 Hardware Replacement Guide