So, I watched an IGN video on youTube and the answer is no. You can only upgrade the SSD the rest of the components are soldered. The steam machine is intended to be kept simple and for the living room, so while you can tinker with software, DIY hardware tinkering is very limited.
https://youtu.be/xb3a3EKwhGQ?si=qeqBJ5Giwo7IqzxV