Which module you edit to add an H1 will vary by which listing template you are using, but look for a module called something like blog title, click on it and then in the right sidebar, scroll to the bottom of the panel and click edit (it might say Edit HUBL source.) From there you look for source code that matches the top of your blog listing template and insert your <h1>Heading Title</h1>.
You will see my blog listing template uses the HUBL variable group.public_title which is the page title set under your blog settings. You can either change that under your blog settings and use that as your H1 or manually enter H1 text between the h1 tags.
Matthew Shepherd
Freelance HubSpot Consultant CRM Consultant | SEO Specialist