Hi
Do you mean that you don't want the header sticky, right?
You can edit this file: root/templates/purity_iii/tpls/blocks/header.php
look for this line of code:
<header id="t3-mainnav" class="wrap navbar navbar-default navbar-fixed-top t3-mainnav">
change it to:
<header id="t3-mainnav" class="wrap navbar navbar-default t3-mainnav">