Keegan here from the Shopify Guru team.
H1 tags can be added to any of your themes files, the main file being theme.liquid or index.liquid (frontpage). The issue with adding these tags in, is they can alter the look of your site (as H1/H2/H3) are all different font sizes for your text. So that means, adding them in, may alter your site in a bad way (make it look out of place, or bad) from its current look.
Suggested way to add H1 tags, if you didn't build the theme yourself, is to contact your theme developer for Retina at email@example.com so they can assist you with getting the tags in, and correctly placed so nothing looks bad. :)
Thanks for your post!