|Ubiquity offering a Ubuntu 15.04 to Ubuntu GNOME 15.10 upgrade|
It's time to upgrade your Ubuntu again. The usual (and recommended) way would be to use Ubuntu’s Software Updater to do the upgrade for you. It handles downloading the right packages for you and makes the upgrade process as smooth as possible.
But there's another way and that's to do the upgrade from a Ubuntu Live session. If you haven't already creating a bootable USB stick is easy.
Upgrading from a live session is a mostly offline process. You already have the packages you need on the bootable media so the Ubuntu upgrade process can just use those. You can optionally let it download newer versions during the upgrade, but if you're in a hurry then you can defer that for later and have a working Ubuntu up and running more swiftly.
Warning: While running the upgrade on my particular setup, Ubiquity got stuck at “Restoring previously installed packages…” which I take to mean something broke while it was reinstalling non-default packages during upgrade. I don’t know if this is just one odd case, or if it’s some bug but my guess would be packages from PPAs causing the problem. It’s a good idea to ppa-purge any PPAs you may have added before starting the upgrade.
You can even switch Ubuntu editions as an upgrade. If you feel like a change of Ubuntu editions but would like to retain your settings, then simply create your bootable stick using your edition of choice (Ubuntu GNOME, Kubuntu, Xubuntu, etc) and use that to upgrade. For my upgrade to Ubuntu 15.10 I'll be upgrading Ubuntu to Ubuntu GNOME. Awesome!
Lastly, it's a safety net. Software Updater works really well, but sometimes it screws up and leaves you with a non-working system. While upgrading from a live session can also screw up, you already have the tools you'll need to save your files and do a clean installation. No need to find another PC, download the ISO, and create bootable media before you can start the rescue operation.
So pick your poison. There's plenty of choice here. I've given Cinnamon desktop a short spin and it's awesome, too. Just pick your desktop environment of choice and go with that Ubuntu flavor. You can't go far wrong.