Modify an Android app's icon using APK Icon Editor
APK Icon Editor repacks an android APK with the icon and name changed. It works on the majority of apps, not all of them.
This way is better than modifying an app's icon from within a launcher as it changes the icon on any phone which has the APK installed and throughout the whole android system.
In this video I assume that you know how to get your hands on an applications APK and then install it. If you have any questions concerning the process please inform me.
I used Duolingo for demonstration purposes.
Update: APK icon editor used to work well in the early days of Android. With modern packages it is more of a hit or miss