Once dimension is created, we can’t change some parameters of it: scale, precision, units displayed.
We can open Dimension Tool Options and change these for NEW dimensions, but both Properties and Control don’t allow to modify existing ones.

The only thing we can do is change these parameters, create a new dimension and then hit 'Apply to all' button in Quick Actions section in Properties (which should be a menu command as well: http://illustrator.uservoice.com/forums/333657/suggestions/49010378) — but what if we don’t need to change ALL the dimensions, but only some?
Another thing is to delete a dimension and recreated it (but what if I have hundreds of these on canvas already?)