Dell OptiPlex G1 - Manual (Page 70)

System Power-Supply Removal (Midsize Chassis): Dell ™ OptiPlex ™ G1 Systems
To remove the system power supply, follow these steps:
1. Disconnect the AC power cable from the back of the power supply.
2. Free the system power supply from the securing tab labeled “RELEASE —>,” and rotate it to the right until it stops.
Press the securing tab to the left to release the power supply.
3. Remove the power-supply mounting screw at the back of the chassis.
4. Disconnect the DC power cables from the system board and the drives.
5. Lift the front end of the system power supply up, and move it to the right to disengage the power supply from the slot in the chassis.
6. Lift the system power supply from the computer.