Obviously, there is no grand evidence to support the claim that affirmations can change your face.
And believing that it can is often unhealthy. Be weary of anyone marketing such a claim to you.
However, this is more complicated than you may believe.
How you see your face is not actually how it looks. People see you differently than you see yourself. Everything you see about yourself is passed through the filter of your brain. Your brain automatically adjusts what you see to match the concepts that you already have of yourself.
This means that you are not seeing yourself objectively. You are seeing yourself based on your past experiences, your beliefs, and your expectations. All of these factors distort your perception of yourself.
The way you see yourself affects the way you act. If you see yourself as unworthy, you will act in ways that reflect that. If you see yourself as competent, you will act in ways that reflect that. Your self-image is like a set of glasses that you put on every time you interact with the world. It affects everything you do.
Fortunately, you can improve the way you see yourself. You can learn to see yourself more objectively. You can change your self-image.
When you improve the way you see yourself, you improve the way you act. You become more confident, more capable, and more successful. You can achieve anything you want in life when you learn to see yourself accurately.