Help changing background images

Im new to wordpress themes and I can’t figure out how to change the images in the background sliders.
EX. Do You want to section and What my clients have to say.

Can I please get some specific directions to do this? Thanks

See this video please from the 5th second: https://athemes.com/documentation/sydney/#styling
It shows where you need to go and there you’ll find the background image option.