Simply swipe left in your app or click on the small icon in the upper left corner to open the menu. Choose the point reproviding purchases.
This function makes it able for you to reload already bought issues. This way, issues you bought at an earlier point with the same email address will be reachable for you too. Before you do that, please type in your email address (you find a detailed explanation here).
Important: The chosen email address has to be the same as the one you used for the order. You will possibly be asked to log in with your Google account and your password. Just be sure to use the email adress you used for the purchased issues as well.