If you need help about using the standard language switchers or creating your own language switchers, please tell us what you are trying to achieve, how it should fit into your theme and what you are seeing.
Steps: You can upload a US flag. To change the flag, please do the following. 1. Go to WPML -> Languages. 2. Press the 'Edit languages' link: https://wpml.org/wp-content/uploads/2017/11/1439193-edit_languages_link_2.jpg 3. In the Language row (you want) change the radio button to 'Upload flag'. 4. Press the 'Choose File' button and upload a YourImage.jpeg image. 5. Press the 'save' button and see that the flag has changed.
You might need to refresh the page to see the change. Hopefully, that will help you to change the flag as you want.