Change my custom header image

The person who designed my website made it possible for me to change the header by uploading a new image. I would do this by going to “posts” and selecting the post that corresponds with my header image. Then I would “set Feature Image.” Once I set a new feature image on that post it would update my header. I have had a header with a fake number because we have been putting the content into the website. However, we are ready to open up the site and I want to update my header with a new image that has the correct numbers. Suddenly, when I set a new feature image on the post, nothing happens to change the header image. I have contacted my web designer but he can take days to get back to me. Does anyone else have any ideas on how to change a custom header?

Here is the site: carrlawutah.com

THANKS!