Boot  select must be enable within BIOS first.
Press F2 during unit boot to into BIOS.

Enable boot select, save it.

Plugin an USB external CD-rom and USB boot disk to the USB ports, then reboot the system.

During the boot time, press F12 to bring the boot menu. Then you should see both items. You can select boot form USB now.

Example Model: Acer Aspire E5-575