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 iOS and Android developers no longer work for Stack Exchange.
Requests to make the app open source have been declined:
Around 90% of the mobile applications are built on top of the [public] Stack Exchange API. The libraries we use are open source and anyone can grab them and use them.
The other 10% of the app however consists of internal things that we are not yet ready to put out to the world and say "here are all the URLs, documentation, description of what they do, go wild". This includes the server endpoints for the feed, user registration, etc. Because of these, and the fact that the rest of the apps are simple data-retrieval and demonstration, the mobile apps themselves are not going to be open sourced.
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 January 2020, bugs resulting from parts of the server backend that are not part of the public API (i.e. the 10% of functionality in the above quote) 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 about that, and the respective fix may also fix the dependent piece of the app.
Aside from this, 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? Can I still download them?
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 [developers for Android and iOS]. 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 apps weren't being actively developed, and server-side issues were only resolved occasionally and inconsistently, it made no sense to keep officially supporting them. As such, they have been formally removed from both the iOS App Store and the Google Play store. The API endpoints that existing installations of the app rely on continue to work, since they're (mostly) the same as those for the public API, but as mentioned above, parts of the server backend that are not part of the public API may disappear at any time, and the public API may change in a way that breaks the app, with or without notice.