Change background image in big title section

Sorry, I am sure this is really simple. In fact I did already somehow change the image in the big title section but now I cant remember how I did it! I am totally new to this whole thing and am trying to put up another image.

The site is https://www.themannfamily.org.uk you can see that I have replaced the default image with an image from the bible. Now i need to change it again. Help please anybody?

Thanks