In my template (fileadmin/templates/index.html) I have inserted the word "test". As you see the view of Templavoilà "test" appears.
I am trying to update the mapping but when I click on save, clear the cache and reload the frontend, nothing really happens. I can't find the word test on the frontend, not even in the source code. What am I doing wrong?

TemplaVoilatemplates are stored in the database. You have to save inside the mapping wizard again in order to update the copy in the database. Clearing the cache will have no effect until then.