I am having the same problem. I opened newstand, pressed the store button, pressed a magazine, Maxim fyi, and the magazine/app info-install smaller window opened up on top of the store window. I installed the magazine/app just fine but now the magazine/app window stayed on top of the store window. I could not find ANY WAY to get the magazone/app window to close (tried finger drags, multifinger swipes, taps, pressing the home button then going back to the newstand store, pressing the grayed out magazine store window area that is outside and behind the magazine/app window). Absolutely nothing I did would close the magazine/app window and let me back into the magazine store to browse.
Here is how I worked around it. At the magazine/app window (or anywahere really), do the multifinger drag downward to get the open app list to come up ('open app list' is that horizontal row of smaller windows with each window representing one of the open apps). Now put one finger on the magazine store window and drag/flick upward. Now touch the newstand window so it becaomes full screen and press the store button. Voile! The magazine/app window is gone and you are back to browsing.
Fyi, (and I am sure many know this already) this one finger drag/flick upward of the small open app windows in is a great way to close some of the many open apps (not that closing open apps is needed, but I prefer to keep the amount of open apps limited).
I hope that helps.
((suggestion to Apple, put a 'back' soft button on the page, or activating back by tapping twice on the screen, or something like that))