Dell OptiPlex G1 - Manual (Page 63)

Computer Cover Removal (Midsize Chassis): Dell ™ OptiPlex ™ G1 Systems
To remove the computer cover, follow these steps:
1. Turn off power to the computer and all peripherals, and disconnect the AC power cables from their AC outlets.
2. Press in on the two securing buttons until the cover is free to swing up.
3. Raise the back of the cover, and pivot it toward the front of the computer.
4. Lift the cover off the hooks at the front of the chassis.
Four plastic hooks on the inside-front part of the cover secure it to the chassis.
Before you replace the cover, fold all cables out of the way so that they do not interfere with the cover or with proper airflow inside the computer.