Even if you could, it is often impractical to even attempt.
First thing you need to do is find out if your current 8400M G is installed in a slot of soldered to the motherboard. A model similar to your ASUS F3Sc showed it's 8400M installed in a slot.
If it is removable, your next step would be to try and find a card which performs better, with no increase in power consumption or requiring better cooling that the 8400M G. Typically, this is where you find the idea fruitless as any upgrades you may find do not give a sufficient boost in performance to warrant the cost.
-Wolf sends