AWS image hosting and connecting to page templates

Hi
I have taken over a site where the images are hosted in AWS S3 and I can not see how to change an image?
I know how to add the image to AWS but where does the AWS URL go?

There are images on the site and I cannot find the AWS URL’s in the liquid codes, I am new to AWS so bear with me.

If I want to change an image on my site, its not just the case of uploading a new image as im using the Flex theme and the way it has been built is with HTML.
I need some guidance please
Thanks