In your Flutter project’s root directory, navigate to .../android/app/src/main/res. The various bitmap resource folders such as mipmap-hdpi already contain placeholder images named ic_launcher.png. Replace them with your desired assets respecting the recommended icon size per screen density as indicated by the Android Developer Guide.
Updating the app icon
IOS.
In your Flutter project’s root directory, navigate to .../ios/Runner. The Assets.xcassets/AppIcon.appiconset directory already contains placeholder images. Replace them with the appropriately sized images as indicated by their filename as dictated by the Apple Human Interface Guidelines. Keep the original file names.