Where do you change the background image (HomeBg-mixer) behind the content blocks

I cannot find where to change the background image (HomeBg-mixer.jpg) that is displayed behind the Udesign content block on my home page.
I can only change it by uploading a new image with the same name trough FileZilla

You should contact the theme/item author for the support. There supposed to be an option to change/upload the background ( you can also refer the documentation )