Dell OptiPlex G1 - Manual (Page 74)

Riser Board Removal (Mini Tower Chassis): Dell ™ OptiPlex ™ G1 Systems
To remove the riser board, follow these steps:
1. Remove the expansion-card cage and remove all expansion cards .
2. Place the expansion-card cage on a flat work surface with the riser board facing up.
3. Remove the riser-board mounting screws.
4. Slide the riser board away from the release handle until it stops; then lift the riser board away from the expansion-card cage.