explore in the bios a little, it's very straight forward, the selection would say something like first boot device, second boot device, third boot device and an option for each.
its usually under something like standard bios options but varies with each bios. and then here should be option that says select boot device... or boot select order... anything like that. it really depends on mobo and bios. gl
or if it doesnt load or u cant find it there is always the F keys. for me its f11 to select which is prolly the most common