Changing the boot order in the Bios is a fairly simple task, that is similair on all computers.
There are several different BIOS manufacturers, and the interface and keys are different for each.
Below is a guide written using Insyde BIOS on a Toshiba Laptop, the basic steps will be the same on other BIOS,
but the keys will vary, although instructions should be displayed on which keys to use.
Switch the computer on and press F2 to enter the BIOS setup. Use the right arrow to move to the boot menu. Use the down arrow to select 'usb' for example. Then use F8 to move it to the top of the list. Finally press F10 to 'Save & Exit' and press 'enter' to save changes.
The computer will then restart and boot from a Usb drive if there is one connected, If there is no Usb drive connected it will boot from the Hard Disc or what ever is second on the boot list.