To setup alert notifications before an items expiry, go to item's edit screen by tapping on the item as shown below. Now make sure an item expiry is set (See this article to understand how to setup item expiry).
Now scroll down to the "Alert" section and tap on the "Alert" row. In the following screen, toggle Expiry Alert to YES and select number of days/hours/minutes before the expiry date that you want to be alerted. For example if you pick 1 day then you would be alerted a day before the expiry date.