Ok 
Is it worth upgrading from 19.10? I only boot Ubuntu from usb to occasionally for banking etc
Is it possible to boot into 19.10 and do the upgrade that way or would I have to mount 20.04 to usb in windows then boot ?
Bear In mind I don’t have it on my HDD or SSD just usb
Unless you’re looking for some new feature from the new version don’t upgrade just yet and give it a couple months.
When you say you’re booting off USB do you mean you boot in “live CD” mode where it doesn’t keep any persistent data or did you actually install it on the USB? In the former case you can always grab the latest ISO image, put it on your USB, if you don’t like it or doesn’t work you can get the ISO for your previous (working) version and put it back on the USB. In the latter a full system upgrade is not easily reversible (will require lots of tinkering with the terminal) plus I suspect installing on an external USB is a relatively weird configuration (when it comes to the boot loader and stuff) and an upgrade might fail in this case even though it would work fine on a normal install.
Put it on usb with the instructions on the Linux website.
Put their usb in, turn pc on and then click “ try Ubuntu without installing “
Got 20.04 set up on usb