how to change frontpage picture on the topsite?


I have the IIldly theme but want to change the frontpage picture on the top?
I can’t do this because there is a lock on it!!!
I read this in jumbotron section:. Will only be displayed if a custom front-page is set in Settings → Reading.
but I can’t change it

And a another problem is I want to change the color of the Testimonials Section. it is now purple and I want a new color!!

so please help me with it otherwise I have to choose e another theme/template