Dell OptiPlex 990 (Early 2011) - Manual (Page 42)

8. Disconnect the 24-pin power cable from the system board.
9. Remove the screws that secure the power supply, from the back of the
computer.
10. Push in on the blue release tab beside the power supply, and slide the
power supply towards the front of the computer.
11. Lift the power supply out of the computer.
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