Because there are several minor incompatabilities between 2.3.5 and 2.4.x, your existing install will not automatically be updated to 2.4.x. If you want to run 2.4.x, change to the emc-2.4 repository by following these instructions:
run System/Administration/Synaptic Package Manager
go to Settings/Repositories
In the list of Third-Party software there should be at least two lines for linuxcnc.org.
For each of them:
Close the "Software Preferences" window
Click "Reload" as instructed
Click "Mark All Upgrades"
If you use a mesa card, find the proper hostmot2-firmware package for your card and mark it for installation. Hint: do a search for "hostmot2-firmware in the synaptic package manager.
Once you have done the upgrade, update any custom configurations by following these instructions: UPDATING
As future 2.4.x versions become available, the ubuntu package manager will prompt you to install them as updates.
To return to emc 2.3.5, go back to the Software Sources window, change emc2.4 back to emc2.3. "Reload". Find emc2 in the package list. Select "Force Version" in the menu. Choose "1:2.3.5". If you have emc2-dev installed, select it and Force Version for it too. Choose "Apply".