Before you do step change the package name, you must do this step for firebase project configuration
Please note : this step is mandatory to generate google-services.json file
Visit Firebase Console
Please register or log in first and then select create new project. In the dialog window enter the name of the project that will be created and the contents of the state and then click the create project to continue.
After successfully creating a project on Firebase then the page will be redirected to the dashboard project that has been made.
Then on the dashboard Firebase select Add Firebase to your Android app. In the next dialog window enter the name of the Android application package and SHA1 of fingerprints Certificate (optional).
Next will download the configuration file json google-services.json automatically, which will be added in the directory app in the Android project.