Change header-bg

Hello ! First, I'm french Wordpress user, sorry for my mistakes :) Maybe there is the answer of my question on the forum but I don't find it. I would like to change the background but not the backgroud where there are messages, I want to change the backgrounf behind the slider and title. Do you understand ? The name of the image is header-bg.ftp and when I change in FileZilla, there is not modification on the website... WHat can I do ? Thanks for your help ! :)