Open the terminal window. Enter the following commands:
gpg --keyserver pgpkeys.mit.edu --recv-key 96935D7D gpg -a --export 96935D7D | sudo apt-key add -Enter the password where prompted in order to run the administrative commands. If you don't perform this step, you will get a warning about "unauthenticated software" when installing bugfix releases.
run System/Administration/Synaptic Package Manager
go to Settings/Repositories
In the list of Channels are two lines for linuxcnc.org. (If there are no entries for linuxcnc.org see [Repository Updating]?)
For each of them:
Close the "Software Preferences" window
Click "Reload" as instructed
Click "Mark All Upgrades"
Once you have done the upgrade, you will need to update any custom configurations by following these instructions: UPDATING