How do I move a button on mobile only using custom CSS

Site URL: https://www.ettiperez.com

Trying to move the 'shop now' button on mobile site only higher up so that it overlaps the background image.

Ideally I also want to remove the space between the background image and the 2 poster images below without having to change the background size to 'auto'

But moving the button up is priority, please help :)

Please post the site-wide password so we can take a look at your issues.