Ok yes it works when I made a new menu, with k2 tag option, and I link it with one keyword and a specific template, it gives me that tag with the modified template. .... I hope you understand what I have written :P
**** EDIT: I found my mistake I have to copy the default template in html/com_k2/template, and modify that tag.php... so long I have made folders with different than the default, thnx for your time!