Never use a stick to update Ubuntu. For most Linux distros, this is the worse option. I am more of an arch Linux user, where pacman -Syu will always work, but
See
man do-release-upgrade
I've never used that command before. I know it's possible however with
sudo apt-get dist-upgrade
but I do not remember exactly what I did, as that was a while ago on an Ubuntu server system, that now runs just plain Debian.
Either way, the stick is generally not the best way to go. It's more complicated and can lead to more.. complications.