I used react-native init MyApp to initialise a new React Native app. This created among others an Android project with the package com.myapp.
react-native init MyApp
com.myapp
I use the react-native-rename* npm package. Install it via
npm install react-native-rename -g
Then, from the root of your React Native project, execute the following:
react-native-rename "MyApp" -b com.mycompany.myapp