If I am not mistaken you do that when you create your community structure>Create New Blog>Predefined Labels
but how to edit the existing structure... I am not sure.
It's easy. Just go to Admin >> Features >> Labels.
The trick is that you must change the scope to a particular board, rather than leaving it at community level.
So in the example below, I've changed the scope to the Interiors board in our Discussion category on Workshop. Then I can add or remove any labels from the list as I see fit.
Hope that helps.