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
very simple way :
cd /your/project/dir
run this command :
grep -rl "com.your.app" . | xargs sed -i 's/com.your.app/com.yournew.newapp/g'
rename your folder
android/app/src/main/java/com/your/app
change to
android/app/src/main/java/com/yournew/newapp