Android and IOS both have their own set of rules for creating app icons. For React Native, since it’s cross platform, but it still has to follow the app icon rules for Android and IOS.

App icon for Android

App Icon file name: ic_launcher.png, ic_launcher_round.png
App Icon file size: mipmap-hdpi, mipmap-mdpi, mipmap-xhdpi, mipmap-xxhdpi, mipmap-xxxhdpi
App Icon file locations:
app/src/main/res/mpimap-hdpi/ic_launcher.png
app/src/main/res/mpimap-hdpi/ic_launcher_round.png
app/src/main/res/mpimap-mdpi/ic_launcher.png
app/src/main/res/mpimap-mdpi/ic_launcher_round.png
app/src/main/res/mpimap-xhdpi/ic_launcher.png
app/src/main/res/mpimap-xhdpi/ic_launcher_round.png
app/src/main/res/mpimap-xxhdpi/ic_launcher.png
app/src/main/res/mpimap-xxhdpi/ic_launcher_round.png
app/src/main/res/mpimap-xxxhdpi/ic_launcher.png
app/src/main/res/mpimap-xxxhdpi/ic_launcher_round.png

To change or update the app icon for Android, just replace the icon images in those image folders.

App icon for Android

App Icon file name: Icon-size.png
App Icon file size: 40, 58, 60, 80, 87, 120, ,121, 180, 1024, etc
App Icon file locations: Images.xcassets/AppIcon.appiconset

To change or update the app icon for IOS, launch XCode, navigate to Images.xcassets folder from the left directory panel, and then select AppIcon, then drag icon images to the appropriate location.

Generate App Icons

For Android, it can be done in Android Studio. File -> New -> Image Asset, or it can be done via a online Android icon generator at https://romannurik.github.io/AndroidAssetStudio/icons-generic.html

For IOS, search for IOS app icon generator and you should find a lot of online tools for generating IOS and Android app icons.

References:
https://developer.apple.com/design/human-interface-guidelines/ios/icons-and-images/app-icon/
https://developer.android.com/studio/write/image-asset-studio