You can tap the F12 key while booting to select your boot device. When you reach the boot device selection screen, you can use the arrow keys to select the device, and hit Enter to boot.

If your USB drive isn't showing up, while you are on the boot device selection screen, try unplugging the device and plugging it back in. This is an issue we are working on resolving with a firmware update. There is also a workaround if you are unable to get to the boot device selection screen.