You would upload it to your images folder in your theme (FTP would probably be the fastest way to do this).
Then edit the .legacy-green .slider-wrap‘s background image by replacing the one that is there with the new one you just uploaded. Example, if your image is named custom-image.png then you would replace “slider-bg.png” with custom-image.png like this –
background-image: url(“images/green/custom-image.png”);