Hard to be specific with a vague question but here goes:
If it is in a block form... enter edit mode - select block to edit - if image block, select "image" on top of window that opened and you'll find "choose image" as an option - choose new image or upload one from resulting link to your file manager.
If it built into your theme... you'll need to open a copy of your theme on your desktop - follow folder path of your theme until you see folder titled "images" and replace offending image in that folder (must be named and sized the same or more edits required in theme to match)and FTP the new folder to your server to replace the original.

There are other methods and circumstances but you need to be specific about what and where on your site (links to site helpful)

Adrian