You have 3 H1 tags, one of them is the site title and it is considered an H1 tag. It's OK .. you can ignore this one because it is tagged (check it) as "site title" in the HTML header section of the page so Google will understand this one.
The other two you must find and fix them. By the way, what is your H1 tag?
If you don't have technical knowledge you can check heading tags with tools like SEO Minion, META SEO inspector.
To change the heading tag you go to Online Store > Actions > Edit Code, then continue to your Assets folder. In that folder, you'll find the theme.scss.liquid where you can change your heading tags. According to SEO, there should be only one <h1> so you can change the rest in h2, h3,...
On which of your pages do you have multiple H1's? Just checked your collections pages (being one of the very important types pages for SEO) and there appears to be just one.
Also, don't be too obsessed with this. Many tools, unfortunately, "over report" errors that are not necessarily and error or a SEO problem. There CAN be good and valid reasons for multiple H1's. In any case, its not a major issue and even fixing this won't change much for you in Google.
If you do want to change it you should be able to do so in your liquid templates used for the page types in question.