Best answer: Maybe. This question depends on which pre-built PC you've purchased. Some system integrators (Dell, HP, Maingear, NZXT, etc.) may use proprietary parts that can make upgrades a difficult task.
[h=3]You can upgrade some pre-built PCs[/h]If you've purchased a pre-built PC with a standard chassis and motherboard, there's a very high chance you can upgrade various parts. If either or both are proprietary, your upgrades may be limited to just RAM, GPU, and storage. It's not something that's easily considered when buying a PC online since not all vendors will provide details or internal photos.
Full story from the WindowsCentral blog...