dual boot - "your system is running in low graphics mode issue"

i dual booted my windows 8 pc with ubuntu 13.10. when i select ubuntu from boot menu then i got an error message saying that "your system is running in low graphics mode".i am using hp pavilion 15-n0t0tx (graphics processor radeon hd 8670m). what is to do?
someone says that reinstall your graphics driver using command prompt but i don't know how to dawnload things from command prompt.i am new at linux. please help me.