The first of the several steps you should take is to find out why your users are leaving the app. Once you find out, you’ll then be able to make the right strategy and re-engage. You can add a feedback option and prompt the users to share some of their experiences. In addition, you can use tracking tools and analytics.
Besides these, you can also use deep linking, build re-marketing lists, offer something unique to understand your most loyal users and make sure you do a proactive outreach through emails and push notifications.