Replacing the graphics card in a laptop is somewhere between extremely difficult and impossible. I've seen external graphics solutions, but have no idea if they are worth the money or even work at all. If there is an internal replacement available you will need to go through Acer to get it.
One thing you may be able to do is to allocate more of your computer RAM to the graphics subsystem. See if there is a place in the BIOS to do that. It won't turn a mediocre graphics card into a Crysis-Killer, but it might help.