Viewport meta tag for mobile screen re-sizing

[Cascade]

I need to put a viewport meta tag in the header:
<meta name="viewport" content="width=device-width, initial-scale=1.0">
It's for re-sizing the text in different devices eg: mobile, tablet etc.
Where do I find the header for html tags please?

[Klemen]

To add anything into the head you will need to modify the GBook templates. Look into file: templates/default/overall_header.php

Open the file in a powerful text editor, such as the free Notepad++ (do NOT use the Windows notepad), find the head tags and add your own code.