Power off your computer and while it is powering on, keep pressing the F12 key to access the boot menu so you can select USB as an option to boot to.
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I understand. Unfortunately, I do not know what you have tried before contacting us.
Are you sure the USB device is a bootable device? What exactly is on the drive?
The download installer tool needs to be run in order to create the bootable ISO file. Did you run the download tool installer and create an .ISO file?
The USB Download install requires that you locate the ISO file when creating a bootable USB.
Did you locate the ISO file?
I am still here -- It is likely that your USB was not properly setup as a bootable device. If it was bootable, it would load after selecting it from the Boot Menu.