You will need to build React Native from source if you want to work on a new feature/bug fix, try out the latest features which are not released yet, or maintain your own fork with patches that cannot be merged to the core.
Prerequisites
Assuming you have the Android SDK installed, run android to open the Android SDK Manager.
Make sure you have the following installed:
Android SDK version 23 (compileSdkVersion in build.gradle)
SDK build tools version 23.0.1 (buildToolsVersion in build.gradle)