Dell OptiPlex G1 - Manual (Page 41)

Computer Cover Removal (Low-Profile 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. Remove the padlock if one is installed.
3. Press in on the two securing buttons until the cover is free to swing up.
4. Raise the back of the cover, and pivot it toward the front of the computer.
5. Lift the cover off of the hooks at the front of the chassis.
Three plastic hooks on the inside-front part of the cover secure it to the chassis.
Before you reinstall 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.