Once the app is published, the package name cannot be changed. If you'd like to change your package name, your only option is to un-publish your existing app, and then re-publish as a new app with the correct package name. 

When editing your app through our website, you may modify your package name under Build Settings. If editing the package name through source code, you may look in ./android/app/build.gradle file, edit field applicationId, and then re-compile.