Hi.
1) Is there a good reason why K2's default items' titles have <h2> tags (I'm not an expert, just asking...)?

2) I would rather set K2 to output items' titles between <h1> tags. How to do that?

Thanks in advance.

Hi,
You can change the html tags by modifing your K2 template file(s).

OK, thanks.
What about my first question: is there some specific reason why K2 outputs titles in <h2> tags?
Could I mess something somewhere forcing K2 to output <h1> titles?

It is not considered a good practise to have multiple h1 tags.
You can have more than one in HTML but too many might hurt your site's SEO.

Make sure to use overrides, getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates, the file you are looking for is the item.php one.