@colin ,
I concur with
@charvie to use Disks first. Go to Menu, System, Disks (if not there, it's Menu, Settings, Disks). Highlight your USB drive on the left and use the menu to select Restore Image. You'll be prompted to select your *.iso file. Then it will make the image bootable on your USB.
As an alternative, you can use Etcher (
https://www.balena.io/etcher/).
Avoid UNetbootin (inconsistent results), and avoid installing something when what you already have installed will work fine.