If I have this doc for example, how can I change the Chapter words that appear when I select the style “Header 1”? I have to write “Capitolo 1”, “Capitolo 2” and so on, but anything I do to change it results in wrong numbers in the index of content (the “Section 2.1” becomes “Section 1.5” and the third chapter will become the new second chapter). How can I do it?
Go to Tools → Outline numbering (Chapter numbering, on recent versions) to change the “separator, before” for each level.
It seems you are going to write in Italian. In that case maybe you’ll find this free book interesting 
Exactly what I was looking for. Thank you.