After update the header file, visit your website home page.. This is important.
Leave this website open in your browser and go back to Google Analytics -(hopefully you have left this open in a tab or different window.. if not click back to analytics
and work your way back to the overview screen of your website tracking
click on real-time overview and you should see this screen with one visitor viewing website ( this should be you if you kept your site open in another tab.. if its zero, go back to your site and click a few pages then come back to this Google Analytics Screen
We now know that GA is installed and working so Let's Go to Webmaster Tools