inside res://android/build
there is a file called AndroidManifest.xml. inside this file, You'll find the following somewhere
<!-- Custom application XML added by add-ons. -->
<!--CHUNK_APPLICATION_BEGIN-->
<!-- Sample AdMob App ID: ca-app-pub-3940256099942544~3347511713 -->
<meta-data
android:name="com.google.android.gms.ads.APPLICATION_ID"
android:value="ca-app-pub-3940256099942544~3347511713"/>
<!--CHUNK_APPLICATION_END-->
Change the value here.