amd r5 m420

so i had an ASUS Laptop X555B that has 2 graphic card AMD r5 M420 512 mb (primary) AMD HD8500M 2048 mb (discrete/switchable) so when im play gta 5 it only detect the 512 mb graphic card which is the R5 One, that mean all of my game use the R5 my first attempt is to change the graphic setting...