With Newsstand open on your iPad, tap the white "gears" pic at the upper right corner of the screen. This will give you three options for deleting old issues.
This is a good thing if you want to get rid of old, unwanted issues. However, you will not gain much added storage; you'll only get a few more MB afer the deletion. The size of the magazines is surprisingly small.
Post here with your results!
The way that my magazine app works is that I tap at the bottom of the screen (while I am in the magazine) and a toolbar pops up. I tap on library at the bottom and all of the issues that I have on my iPad appear. I tap the Edit button in the upper right and then tap on the X on the magazine issues to delete those issues.