Newsstand is not an app. It is a special folder in which newspaper and magazine apps reside. It is a place to keep all of your periodical subscriptions organized. Since ts not an app, you do not close it. You exit by tapping the home button. Then you can close the periodical app in the same way that you close any app on your device. In iOS 8, it is done by double tapping the home button and then swiping up on the app preview thumbnail.
if your device has frozen on the Newsstand, reset the iPad by holding down on the sleep and home buttons at he same time for about 10 seconds until the Apple logo appears on the screen. Let go of the buttons and le the iPad start up.
EDIT - I just did some experimenting and I was able to swipe the Newsstand preview up off of the screen like any other app that I want to close. I quit using Newsstand when I dropped my subscription to SI several months ago. Perhaps the ability to close Newsstand is relatively new? An iOS a 8.1.3 feature?