I don't know where to change my Wordpress header! Please help me! It's for my father's business : Firstly, i'm not a native speaker so please forgive me if i made any grammar or spelling mistake!

I'm just a wordpress user, actually i've never used wp before until my dad give me his company's website, and i have a huge problem now that i can't find where to change my header image

Here's the website

[enter image description here][1]

And themes customization

I did a lot of researches, and i saw that there's an option called "header" in the customize section, but as you can see i can't find it!

I know it's stupid but i really don't know anything and i'm really confused! 

Please help me, many thanks to you, please! I need it, 'cuz i'm an IT student and they always look at me like i know things, but not that much, it's especially i've only learned C++ not this, i have no ideas what's going on, but i'm afraid that they're gonna judge me and my father!
Please help, thanks, thanks, thanks a lot!


  [1]: https://i.stack.imgur.com/S5qQI.jpg