run System/Administration/Synaptic Package Manager

Settings/Repositories

In the list of Channels ["Software Source" in breezy] are two lines for linuxcnc.org.
For each of them:
  Select and click Edit
  on the Components ["Sections" in breezy] line, change emc2 to emc2.1
  Click OK

Close the "Software Preferences" window

Click "Reload" as instructed

Click "Mark All Upgrades"

Click "Apply"