See link below if you need instructions to remove the Top/Rear chassis lid.

Removing Chassis Covers (TS2-185671979)

Removing PCI-e Riser

Step 1 - Remove two screws from rear of chassis

...

Removed PCI-ie riser from mainboard.

Installing PCI-e Riser

Depending on the size or dexterity of your hands, you may want to review these steps first, plug in the PCI-e GPU power cables first while hovering the PCI-e riser assembly so you do not attempt to re-plug the power cables with a small clearance.

...

Step 5 - Align and tighten internal knob screw

Step 6 - Replace the two screws to rear of chassis