I would like to be able to populate an drop-down extra field on selection of another drop-down extra field.
For example, select a region in drop-down 1 and only the towns only in this region will appear in drop-down 2.
Is there a K2 extension to do this or could you propose the piece of code to add (and in which K2 files to add it, I am not an expert!)?
Thank you!

Hi. You cannot do this by yourself since you are not an expert. One thing you can do is to be more specific about what you want to achieve so other members of our community can point you to the right extension. For example you do not mention where the drop-downs come from.

I would like to have two dropdowns, the first with regions (group) and the second with prefectures (subgroup).
When the user selects a region in the first dropdown, then the second one displays for selection the prefectures that belong to the spesific region filtered through the list of all prefectures.
I suppose I would manually insert the data for the two dropdown extra fields and link each region of the first with a group of prefectures of the second.

It´s necessary to be an expert ? I can not do it too...

No you cannot do this by yourself. You need to know javascript for this. You can hire a developer to do it for you.