Dell OptiPlex GX1p User Manual page 56

Riser Board Removal (Midsize Chassis): Dell™ OptiPlex™ GX1p
To remove the riser board, follow these steps:
1. Remove the expansion-card cage
2. On a flat work surface, turn the expansion-card cage over so that the riser board faces up.
3. Remove the riser-board mounting screws.
4. Slide the riser board down until it stops; then lift the riser board away from the expansion-card cage.
and remove all expansion cards.