change text on 'not found'

Hi, this is my last post for today.
I have so far been able to adjust and costumize the theme well, but I could't change the text on the not-found-page.
I put in new text where the English sentences were in the editor > not_found.php, but it doesn't show on the site (not even after refreshing). Any idea?

Thanks a lot in advance,

Ilke