Dell OptiPlex 3000 Tower Service Manual page 56

Steps
1. Lay the computer on the right side.
2. Disconnect the power cables from the system board and unroute them from the routing guides on the chassis.
3. Remove the three (#6-32) screws that secure the power-supply unit to the chassis.
4. Slide the power-supply unit away from the back of the chassis.
5. Lift the power-supply unit off the chassis.
56 Removing and installing components