That is the original and correct way of the HTML that the theme uses. The site title is H1 and the post titles in the listing are H2. If you want to change that you need to customize the theme.
Depending on which section of the website that you want to change the code you will need to override a different file of the theme. You need to install the Child Theme and read the articele below:
The file that is responsible for the header section of the posts should be copied from:
and there you can change the h2 to h1 on line 15.
Please consider that this is considered as a customization and it is outside of our support scope