But what is it that you need to change? If you have a small number of beta users, you can just text them to ask. There are also tools available to include some sort of survey, but most people consider them as annoying. It might help if you offer them specific incentives (a free purchase, for example). They could be digital incentives, such as the well-known badges (gamification) or real-life incentives. If you are interested in the latter, checkout Kiip at http://www.kiip.me/developers for some examples. They have a great SDK that you can add to your app. For example, it enables you to offer a free cup of coffee to your user if he has fully completed his profile.
If you want to know who your app users are, you need to have additional information about them, such as their locations and what they expect from your app. It is also interesting to know something...