My Lenovo C205 is an "all in one PC" which means it doesn't have a case. I'm thinking about getting a new GPU but I'm not sure how to open this thing up when it doesn't have a case. Please help.
It's pretty unlikely that you can replace the graphics card in that computer. Usually laptops and all-in-ones like that have non-standard GPUs that may even be built into their motherboards to meet their unique size/form factor requirements. At any rate, you wouldn't be able to just stick in a regular AMD or NVIDIA card.
Considering the C205 has no PCI slots to speak of. You are stuck with what you got. There are USB options, but those are not very powerful, so no gaming, and likely no HD video.