You can change css styles of headings tag from h1 to h6 in Jupiter X.
In order to change styles of heading tags:
1. From the WordPress left dashboard, go to Appearance > Customize or click on Customizer from Wordpress Admin bar.
2. From the Customizer left dashboard, go to Font & Typography menu.
3. In Typography Tab, select Customize for the Heading tag you want to change its style.
4. Change the heading tag's css style properties.
5. Click on Close and Publish the changes.