Upgrading an AMD APU is easy. If you can insert a plug into a wall outlet/main, then you shouldn't have any problems with that. There are how to videos on youtube.com you can watch to see how it is done.
A video card might be a problem. It will depend on whether you have an available PCIe slot on the motherboard and the size of the case. The has to be enough clearance for a card to fit inside the case.
Did you mean you are currently using the embedded video in the APU and want to add a card for CrossFire or do you mean you currently have a seperate video card and want to add a second card? Either way a video card might be a problem. It will depend on whether you have an available PCIe slot on the motherboard and the size of the case. There has to be enough clearance for a card to fit inside the case. The easiest thing to do is open the case and take a look. In addition check the owner's manual.