How to change the header image?

Hi,

Sorry if my post has already been replied on previous posts.
I am trying to change the main header on the home page (the banner with a guy on it) by my own image. I have tried most of the advise from previous posts however nothing is happening.
I must say that I have no knowledge/skills on coding so I am struggling in making my website.

How can I upload my own image replacing the one with this dude?

If any one has a good expertise on the job board and would be happy to mentor me, please let me know as I am so struggling creating my website !

Thanks for your help!