Since I have already downloaded the file at work, I would like to use it instead of upgrading it using the software-updater option.
If you used one of the tools for putting an Ubuntu image onto a USB stick to boot from, then it would work the same as a CD.
It is possible however to boot up into an install disk/USB and choose a USB stick as the target for an install just like if you were installing to a hard drive.
Do not use it, because that particular method is unreliable: it may erase not only the old Ubuntu but all other operating systems (Windows, other Linux distributions, ...) and data partitions on your hard drive as well. Then use this fine disk partitioner, to completely destroy the Ubuntu partitions. All actions you do in Gparted have to be confirmed by a click on the Apply button in the panel (the green tick V), before they are being executed. Note: don't format the Ubuntu partitions, just destroy them. If you wish to support my website, you can configure your ad blocker to make an exception for this website. If you want to save your e-mails, copy this directory to the USB memory stick. Reboot anew from the DVD and let it check itself for errors. When no errors are found, boot your computer from the new Ubuntu DVD and choose "Try Ubuntu without installing". Establish internet connection and start the installation by clicking the desktop icon of the installer. During installation, the best choice is now manual partitioning.
This unreliable method might do this in spite of the fact that it promises to erase only the old Ubuntu.... Ubuntu itself will be placed on the root partition: give it the mount point "/" (without the quotation marks).
If you boot from a live CD or DVD (non-rewriteable) and then run sudo apt-get update and sudo apt-get upgrade or tried to install a package that isn't already on the CD what would happen? If you wish to boot a Live CD and make permanent changes on an existing installation already on a partition, you can 'chroot' and then run the commands.