Do you mean USB?
If I start my computer and hold down F12 it asks me to choose what to boot from-in which case I select a number. If I go into the BIOS menu, which is an option also shown on the same screen(and this can also be accessed by holding F2 when starting)and open the system list then go to boot sequence and enter and follow the prompts to change the order.
If I'm wrong someone correct me. Otherwise I hope this helps.