Insert that into your theme.liquid file (online store > themes > click '...' on the theme you want, then 'edit code'). Insert it at the bottom of the file, right before the '</body>' tag.
Alternatively, you can also use the appEasy Editsto make the edit yourself without touching any code, just click on the section and set the top margin to 0. You can the edit and delete the app without paying for anything, and keep the edits.
Fair disclaimer, Easy Edits is an app I've worked on. If you need any help inserting the code or using the app, just let me know!