A little information about Google DNS can be found here: https://developers.google.com/speed/public-dns/docs/performance
Firstly open up the Settings menu on your iPhone/iPod/iPad/iDevice the menus should be pretty similar on most of the devices 
Now select WiFi if you are connecting via WiFi mostly
Now you’ll see DNS is currently default on this device and it searches from the router settings for DNS, we shall now change this.
Clear the DNS entries and enter 8.8.8.8, 8.8.4.4 as below
Save and refresh you’ll notice you might be disconnected and reconnected very quickly…
Now if you check you are using Google DNS for your iOS device!
Now many say if you restart the device OR enter airplane mode for a few seconds and return to normal it will flush the DNS cache on your device.
Best to hit up Safari as well and enter the settings and clear history/data/everything