A good way to start would be to first define metrics to identify who are your happy users, and engage with them to let them know you care about adding value to their experiences.

For simplicity, start by looking at your most engaged users (e.g.:spent over 10 minutes per day in your app, for 5 out of the last 10 days). Reach out to these users to understand (a) what they like about your app, and (b) how the app could delight them further. Offer them a promotion/reward for helping you with feedback. They will see you are keen on keeping the relationship going and feel part of any success you have.

The same could also work for your most unhappy users. Ping users who aren't engaging with you, have left you bad feedback, to find how you can make yourself relevant to them, and what they are missing out on. You may be surprised to see how, once you start listening to them, they can become your most vocal evangelists.