Change boot priority order?

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.

Quite regularly I boot into Ubuntu from an external USB drive.

I can press "F12" at startup and choose to boot from the "+USB Storage Device" there.

The ability to press "F12" passes really fast though and the opportunity is often missed,

leaving me to go through the Windows 7 boot up procedure, then a restart to try again.

I would prefer to be able to set this boot order in the BIOS settings.

Please can you tell me how I can do this?