How do you change the “Anspress” cover photo on profiles?

I’m referring to the pick “Anspress” background image. I saw your response to someone else when they asked a similar question but your solution didn’t resonate with me. And that person did not respond so I’m not sure that it resonated with them either. I am not very tech savvy so please offer a very detailed response. This is the previous response I am referring to. Thank you and I appreciate your time.