Dell OptiPlex 990 (Early 2011) - Manual (Page 54)

9. Remove the screws that secure the system board to the chassis.
10. Slide the system board towards the front of the computer.
11. Remove the system board from the chassis.
Installing The System Board
1. Align the system board to the port connectors on the back of the chassis and place
the system board in the chassis.
2. Tighten the screws securing the system board to the chassis.
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