Hello @Jerry Lipan ,
Please open your iOS project, there is an item named "Asset Catalogs" which manages and groups all version of a given image asset required by an app, double-click "Assets" to open it in the editor, you can see the default icons under "AppIcon". Then you can click the ** ... ** button in the upper right corner of each item and replace the default icon.
Pay attention to the required type/size when designing and replacing the icons. For more details, refer to Application Icons in Xamarin.iOS - Xamarin | Microsoft Learn
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