Meta tag viewport in T4_Blank Template?

Meta tag Viewport. How can I override it in T4_Blank Template?

The base file is: ROOT/plugins/system/t4/themes/base/html/layouts/t4/index.php

If you've not overridden this file in template yet, you can copy above file to template folder:

ROOT/templates/t4_blank (your default template)/html/layouts/t4/

and customize there.