Changing the Endless.exe Icon usually results in a corrupt file.
Although if you create a custom client and have an installer, depending on the installer you can give endless a custom icon using a command line for a shortcut.
Take 'Smart Installer' for instance. You can choose to place a shortcut on the desktop and then choose a file.ico to use for the shortcuts icon, which you then place along with the installation files in the root folder.
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