Android

Step 1 - Go to android\app\src\main location and find the 'AndroidManifest' file change the package name with new name.

package="com.example.newsapp">

Step 2 - Go to \android\app location and open build.gradle file where you have to find the 'applicationId' change with new name.

applicationId "com.example.newsapp"

iOS

Step 1 - Go to \ios\Runner location and open the 'infor.plist' find this below line

<string>$(PRODUCT_BUNDLE_IDENTIFIER)</string>

change it with your new package name 

<string>$com.example.newsapp</string>