Ask HN: Resources to learn how to maintain a web project?

I have a few side projects that are picking up a bit of traction, and now I'm afraid to make any changes because users have entrusted me with their data. There are plenty of resources that teach you how to build and deploy an application, but I haven't found anything about the best ways to maintain a running app with active users (bug fixes, new features, migrations, etc). Any recommendations?