How to change Android App Name and ID of an existing application?

Actually you need to change the name in several places:

First, as you said, the name of the string, which is the visible name of the application.

Second, go to activity->src and right click on the package (com.example.whatever) and do refactor->rename;

Then go to the manifest.xml: and change the field in:

<manifest package="com.example.whatever" >

If you are using native code, JNI, you will also have to change the names of the c++ functions, which is a pain in the ass:

Java_com_example_whatever_activity_function()

For those who aren't using Android Studio and want to do it manually (e.g. if you're using React Native), I just recently went through this and had to change it in the following files:

index.android.js
android/settings.gradle
android/app/build.gradle
android/app/src/main/AndroidManifest.xml
android/app/src/main/java/com/<app id>/MainActivity.java
android/app/src/main/java/com/<app id>/MainApplication.java

Package name (in java).

The app name is also in the manifest, although I don't think that needs to be unique, but still would be good to change it for clarity.