This is an accepted solution.
Eunice here from Shopify, thanks for posting, and welcome to the Community!
The sticky header is a feature of the Narrative theme, so while there isn't the function to disable this from the Shopify admin, there might be a chance it can be disabled and/or customized by the Shopify Themes team.
Please visit the Help Center and log in to your account to connect with a Shopify Support Advisor via email, callback, or chat. Once you're connected, an Advisor will take a look at your storefront and let you know if this is eligible for customization.
Alternatively, if you feel comfortable with editing code, I did find this Shopify Community post containing a tutorial to remove the Narrative sticky header. Please note that this tutorial is not supported by Shopify, which means it is outside of the scope of our support, as outlined in our Design Policy. With this said, you are more than welcome to follow the guides and attempt these changes on your own! We typically recommend duplicating your theme and creating the changes on the duplicate, so that the live store is not affected while making these changes.
Hope this helps!