If you had this theme how would you change the banner?

Theme —> https://www.theme-time.com/26/

Some people call them banners and some call them headers, either way I just want to change it. You see, I have my own design in mind and need to change the current one but I don’t know how. I’ve tried but I always get an error or something. I guess it would need to be the same size or something but I have no way of figuring out how big or small my banner needs to be. So if anyone could just take a few seconds out of their time to help me out, I would be ever so grateful.