How to downgrade Ubuntu Linux system to its previous version - LinuxConfig.org

The Ubuntu system downgrade procedure is relatively straightforward. The trouble only may arise during and after the system downgrade. In this guide, we will be downgrading Ubuntu 18.10 Cosmic Cuttlefish to Ubuntu 18.04 Bionic Beaver.
This is a companion discussion topic for the original entry at https://linuxconfig.org/how-to-downgrade-ubuntu-linux-system-to-its-previous-version

After downgrade no audio driver and i try to fix this issue by reinstalling linux kernal extra to auto install all missing drivers. but i can get these error message

sudo apt-get install --reinstall linux-image-uname -r

Reading package lists… Done
Building dependency tree
Reading state information… Done
Package linux-image-4.18.1-041801-generic is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package ‘linux-image-4.18.1-041801-generic’ has no installation candidate

sudo apt-get install --reinstall linux-image-extra-uname -r

Reading package lists… Done
Building dependency tree
Reading state information… Done
E: Unable to locate package linux-image-extra-4.18.1-041801-generic
E: Couldn’t find any package by glob ‘linux-image-extra-4.18.1-041801-generic’
E: Couldn’t find any package by regex ‘linux-image-extra-4.18.1-041801-generic’

How to fix this error?

Just a simple reminder for people downgrading from 18.10 to 18.06. You must reboot the system after downgrading, otherwise you can’t launch any applications.

Kevin,
First, I would like to thank you for posting this how-to. It has saved me hours of having to reinstall from scratch a Digital ocean droplet which I mistakenly created with Ubuntu 18.10, instead of the 18.06 LTE release. The upgrade worked like a charm, and now I have a stable Ubuntu 18.06 LTE VPS that is running like a charm.

For others who have to do this on digital ocean, here is a little addition to Kevin’s how to:
If you reboot immediately after finishing the update, you will lose the network connection and will not be able to update to the latest release (not sure why). Instead, just before the final reboot, remove or empty the “/etc/apt/preferences” file and execute apt update, apt upgrade, apt dist-upgrade sequence, which will get you the latest versions of kernel and other installed packages, and only then should you restart the droplet.