Change Logo

Make sure you have 1024*1024 png logo file ready.

Go to the http://appicon.co/ and generate the app icons for both android and iOS.

Change Android Logo

Go to android > app > src > main > res > drawable and replace the ic_launcher.png with 192x192 size logo image

Then go to android > app > src > main > res and replace the folders named mipmap-XXXXX

You are all set.

Change iOS logo

Open the iOS folder in Xcode and navigate to Runner > Assets.xcassets and paste/dragdrop the icons generated for iOS earlier.