How to boot from USB Storage device?

My BIOS is version A16 and the computer Inspiron 7720.

When I go to the Boot Priority section of the BIOS settings it shows that "Legacy Boot" is enabled.

Following the on screen instruction I use the Down Arrow to select from the list:

"+USB Storage Device"

I then attempt to move this list option to the top of the list using the "+ / -" keys.

There is no response. The item does not move up in the list.

Please can you tell me how I can make this work?