I don't know if it voids warranty if you open your your all-in-one. Also, you have to check your manual if there is a layout of inside your computer. Then you have to measure how big your graphics card is and if there are PCI-E slots. I am assuming you can't install anything in there, possibly changing RAM is an option for you.
Your graphics is integrated meaning it is on your motherboard and you cannot remove it.
I have a Acer Aspire all in one Z3760. I Have been able to introduce new graphic cards to it. So that you can be able to upgrade graphic you need to purchase a graphic riser that will give you ample room to install next gen graphics without losing the Big HD screen. Mine cureently has a GT Geforce 730. you need to mold the case at the back though to make room. I plan to install a liquid cooler rather than a fan. These models have good display especially for gaming. I am experimenting whether a new mother board can be installed. All I know is that the Z3760 has been somewhat designed to take upgrades because there is surprisingly ample space that you can work with.