How to Edit Background Theme Photos ?

Hi there!

I am trying to edit the theme photos for spririt (the big background photos in the heading), but I am not sure how to. Is there a better guide on how to use the theme and make edits?

Best wishes,

Rosi

PS This is the guide I am using that is not specific enough: https://premium.wpmudev.org/wp-content/uploads/2015/01/Upfront_guide.pdf