react native change app icon

Add an application launcher icon with automatic sizing. (Android studio)

Go to menu File* → New → Image Assets → select launcher icon → choose image file

It will automatically re-size. Done!
  
################ OR ##################  

Android Icons
Replace ic_launcher.png in folders 
[ProjectDirectory]/android/app/src/main/res/mipmap-*/.

72*72 ic_launcher.png in mipmap-hdpi.
48*48 ic_launcher.png in mipmap-mdpi.
96*96 ic_launcher.png in mipmap-xhdpi.
144*144 ic_launcher.png in mipmap-xxhdpi.
192*192 ic_launcher.png in mipmap-xxxhdpi.

For this I found this tool most useful.
1. Upload a image.
2. Download a zip.
3. Extract into your project.
4. Done

Tool Link: http://romannurik.github.io/AndroidAssetStudio/
https://easyappicon.com/

reference:
https://stackoverflow.com/questions/5350624/set-icon-for-android-application/11845815#11845815