Hi
You can view additional information for the 800-200 series chassis and the Odense2 motherboard at this HP Support site.
Note, you will need Flash to view contents at this HP Support site.
The PCIe x1 slot should be unoccupied.
It looks like there is room for three (3.5 inch or 2.5 inch (with an adapter)) storage devices.
The MB has three SATA ports. Two are occupied (HDD- optical drive).
You can install any compatible device in the unused x1 slot. What upgrade are you looking to do? Sound cards, or M.2 NVME adapters, or network/WIFI cards, or additional SATA port adapters will work.
Tom