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've used the react-native-rename package.
In terminal run the command to install:
npm install react-native-rename -g
In the root of your React Native project, run the following command:
react-native-rename "NewNameOfApp" -b com.companyname.newnameofapp