Dear stsorrell,
Thank you so much for using our theme Mercantile. To unstick header, this feature is available only on the premium version of this theme. So we recommend you to check the premium version for added features. If you need it on this free version, you need to customize the theme codes. You can check custom.js file and customize the sticker header code as your need.
Please try once and let us know the results.
Regards