Assign Category to multiple already existing Discussions

I am dealing with Community Sites in Sharepoint 2013. I have a Discussion Board with multiple discussions in it, all of them with a blank Category field associated.

I've created a Category and I would like to assign / attach this attibute to all the discussions in the discussion board. I cannot find any reference to this, is there a quick way to proceed?

One more thing... Is it normal that the default list "Discussion List" in the site is empty, even if the discussion board contains multiple elements?

Thanks!! M.