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

These screws locks in the riser to the rear of the chassis.

Step 2 - Unplug all PCI-e power cables from Power Distribution Board

Each connector is clipped into the receptacles. One at a time, press down on each connector's clip release before pulling cables from their respective receptacles.

Step 3 - Loosen internal knobscrew

Step 4 - Lift riser from marked yellow lift zones

Use 3-4 fingers under longer yellow lift zone, 2 under shorter yellow lift zone, and then focus lift force towards the slot.

Note: This takes a considerable amount of force.

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 1 - Align the PCIe riser internally

The PCI-e riser has a small protruding metal flap that should align internally to the chassis as below.

Step 2 - Align the PCIe riser's rear

Next to the smaller yellow lift zone, there is another metal flap from the PCI-e riser that should align past where the mainboard gives clearing for it. You will see the other, smaller protruding metal flap that helps guide this end.

Step 3 - Align the PCI-e gold contact segments to mainboard slot

Step 4 - Press down on the riser

Unlike removing the riser, slightly, you will need to focus the force of pressing down on the PCI-e riser directly over the slot.

Note: This also takes a considerable amount of force.

Step 5 - Align and tighten internal knob screw

Step 6 - Replace the two screws to rear of chassis