With some very limited exceptions, no.
First off, many platforms just don't allow this, even if you could. iOS most certainly. If your app is released in the public app store, there is no way to change compiled code after it is deployed, and even if you could, it is not permitted.
You certainly can download new content to the database, though, and make use of that new data. That data might even include some details of layout.
But if you are changing Ruby code, templates, etc. all that is compiled, and replacing already-compiled parts of an app is on each platform either difficult or not permitted. So, you need to re-build and re-deploy, as well as go through any required approval process.