There's a menu option to select the bios. Start your game, open the retroarch menu (quickmenu), then go to "Core Config", there's a bios option in there.
I might remember wrong where it is. I hate the Retroarch menu system. It is impossible to find an option in that convoluted mess, so please look around a bit until you find. Retroarch is great, but a total UI nightmare. Completely useless and obscure options are right next to important ones that people need all the time.
Advertising