I think you have to read the blogpost that was written when NS 6.8 was released: http://www.nethserver.org/nethserver-6-8-final-released/
It gives you all info needed to upgrade. But if you have applied alle updates from software center, you probably are already running 6.8. It should say so in the Dashboard.
In the dashboard is 6.7 (final), and the software does not update.
Previously, I had the 6.6 installed, the update to 6.7 ran without problems, but it never goes on …