How to customize the  tag I have downloaded nexus free theme and I want to change the title
in all pages of my site. 

Is there a way to do it editing the template.php?

thanks...
If you using D7 and not D8, take a look at page_title module. I just googled it for you.


  The word "title" is a bit overloaded. Every piece of content in Drupal
  has a title, and so does every page. The page title is the one found
  in the HTML head inside the  tag. It is also used on SERPs
  (Search Engine Result Pages) and can greatly enhance your websites SEO
  (Search Engine Optimization).
  
  This module gives you granular control over the page title. You can
  specify patterns for how the title should be structured and, on
  content creation pages, specify the page title separately to the
  content's title.