Header Image Will Not Update

When I originally created my website, I designated a header image to one of the pages. A few days later, I tried to change the image, but the header will not change. I was finally able to delete the original image, but whenever I upload a new one, it will simply not appear on the page.

I have the same problem with header text. If I try to change the font, font size, or anything relating to the header, it will not budge.

I am not savvy with coding, so it could very well be a simple fix, but I don’t know where to begin.