This thread is resolved. Here is a description of the problem and solution.
Problem: How to change the flag icon?
Solution: This is pretty easy. You can change flags from, wpml -> languages, then click on Edit languages link.
This action will redirect you to Edit languages interface, under flags column you can upload custom flags.
or follow the steps explained here: https://wpml.org/2019/01/choosing-the-flags-for-your-sites-languages/
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