TL;DR: Unfortunately, you can't.
Thank you for taking the time to improve the Stack Exchange platform. Sadly, the mobile apps are no longer in development. To quote one of the Stack Exchange developers:
We are not providing any further support for the mobile apps. There will not be any more builds made for either the Android or the iOS app (even for breaking bugs). We encourage all mobile users to use the responsive website. We hope to continue to incrementally improve the user experience for mobile users.
Also, the Android developer doesn't work for Stack Exchange anymore and the iOS developer has been assigned to another team.
Requests to make the app open source have been declined.
But I noticed some recent changes in the app!
The new close reasons (and further back, the new comment flag reasons) have been changed recently, so they fixed something. Can't they do something about my problem as well?
The mobile apps use the Stack Exchange API to communicate with the server/central database. That means a good deal of the content shown in the app is dynamic and does not require a new version of the app. Unlike the apps, the API is still supported, because many other community applications and projects rely on it.
The mobile apps have not been updated since 2017, so any changes since then have been purely server-side. As of the official discontinuation announcement in 2019, bugs resulting from parts of the server backend that are not part of the public API (e.g. the hot questions list) are no longer fixed.
Does that mean I can't ask questions about the mobile app anymore?
No, support questions can still be asked, and perhaps the community here knows a workaround for your bug.
If you can demonstrate that your issue also occurs in the public API, you can file a bug with that, and probably the fix made to that will also fix the same part in the app.
Aside from that, occasionally, there have been a few cases where a Stack Exchange employee has fixed things server-side specifically for the app, in their spare time. But please don't raise your hopes too high…
Why aren't the apps developed anymore? Why are they still offered for download from the app stores?
Stack Exchange has decided to focus on a single experience for all users, the responsive website. To quote a Community Manager:
Mobile app development requires a special skillset. We no longer have a dev for Android and our iOS developer is working on the web platform. We have a relatively small development team and supporting the apps, which have been a relatively small amount of our traffic, is considered less vital than getting the site converted to be completely responsive and work on new features that have been sorely needed for years. Because the apps have been lying fallow so long, we'd practically have to start over with them (from what I understand).
As the developers weren't working on the apps, and server-side issues were only resolved occasionally and randomly, it made no sense to keep support for the apps. The apps
are still offered for download by popular demand*, to support features like phone notifications which are currently deferred for implementation in the responsive site.
* The Android app has been temporarily removed from the Play Store, but the team says that that's a mistake and they are working on putting it back up.