First questions first. Why would you want to restart an app? Couple of reasons for that.

When you switch from one app to another, the apps do not close, but run in the background. By shutting down apps you use only occasionally, you can free up memory and improve battery performance. Sometimes, an app stops responding, and you may need to restart it.

Here is how you do it:

1. Double-tap the home button. This pulls up the multi-tasking view with screenshots of the apps – three at a time. 
2. Swipe till you find the app you want to shut down. 
3. Swipe upwards on the screenshot of the app to shut it down. You can also use three fingers to simultaneously shut down all three apps (try it, its fun).
4. When you need to restart the app again, tap the home button, and then tap on the app to open it. 

Hope this helps!