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Look in the application's AndroidManifest.xml file for the <application> tag.
This application tag has an android:icon attribute, which is usually @drawable/ic_launcher. The value here is the name of the launcher icon file. If the value is @drawable/ic_launcher, then the name of the icon is ic_launcher.png.
Find this icon in your resource folders (res/mipmap-mdpi, res/mipmap-hdpi, etc.) and replace it.
A note on mipmap resources: If your launcher icon is currently in drawable folders such as res/drawable-hdpi, you should move them to the mipmap equivalents (e.g. res/mipmap-hdpi). Android will better preserve the resolution of drawables in the mipmap folder for display in launcher applications.
Android Studio note: If you are using Android Studio you can let studio place the drawables in the correct place for you. Simply right click on your application module and click New -> Image Asset.
For the icon type select either "Launcher Icons (Legacy Only)" for flat PNG files or "Launcher Icons (Adaptive and Legacy)" if you also want to generate an adaptive icon for API 26+ devices.