How to Upgrade from Ubuntu 18.04 LTS to 18.10 'Cosmic Cuttlefish'

One day left before the final release of Ubuntu 18.10 codenamed "Cosmic Cuttlefish". This is how you make the upgrade from Ubuntu 18.04 to 18.10.

Upgrade to Ubuntu 18.10 from 18.04

Ubuntu 18.10 has a brand new look!
As you can see from the screenshot, a lot has changed. Ubuntu 18.10 arrives with a major theme overhaul. After almost a decade, the default Ubuntu GTK theme ("Ambiance") is being replaced with a brand new one called "Yaru". The new theme is based heavily on GNOME's default "Adwaita" GTK theme. More on that later.

Upgrade from Ubuntu 18.04 LTS to 18.10
If you're on Ubuntu 18.04 LTS, upgrading to 18.10 "cosmic" is a pretty straight forward affair. Since 18.04 is a long-term support (LTS) release (meaning the OS will get official updates for about 5 years), it may not prompt you with an upgrade option when 18.10 finally arrives. 

So here's how it's done. Disclaimer: back up your critical data before going forward. And better don't try this on mission critical machines. You're on LTS anyway.
  • An up-to-date Ubuntu 18.04 LTS is the first step. Do the following in Terminal.
$ sudo apt update && sudo apt dist-upgrade
$ sudo apt autoremove
  • The first command will check for updates and then proceed with upgrading your Ubuntu 18.04 LTS with the latest updates. The "autoremove" command will clean up any and all dependencies that were installed with applications, and are no longer required.
  • Now the slightly tricky part. You need to edit the /etc/update-manager/release-upgrades file and change the Prompt=never entry to Prompt=normal  or else it will give a "no release found" error message. 
  • I used Vim to make the edit. But for the sake of simplicity, let's use gedit. 
$ sudo gedit /etc/update-manager/release-upgrades
  • Make the edit and save the changes. Now you are ready to go ahead with the upgrade. Make sure your laptop is plugged-in, this will take time. 
  • To be on the safer side, please make sure that there's at least 5GB of disk space left in your home partition (it will prompt you and exit if you don't have enough space required for the upgrade). 
$ sudo do-release-upgrade -d
  • That's it. Wait for a few hours and let it do its magic. 
My upgrade to Ubuntu 18.10 was uneventful. Nothing broke and it all worked like a charm. After the upgrade is done, you're probably still stuck with your old theme. Fire up "Gnome Tweaks" app (get it from App Store if you already haven't), and change the theme and the icons to "Yaru".