Some Divi modules have the title text already defined. Those title texts are set to use different heading levels:
Blurb module - the title uses the H4 Heading level.
Call to Action module - the title uses the H2 Heading level.
Circle Counter module - the title uses the H3 Heading level.
Comments module - the title uses the H1 Heading level.
Countdown Timer module - the title uses the H4 Heading level.
Email Opt-in module - the title uses H2 Heading level.
Etc.
Change the default Heading level to a different one
Open the module settings.
Go to the Design Tab → Title Text → Title Heading Level.
Change the default Heading level by selecting the desired Heading level.
Optional: change the styles for the new Heading level.
Customize the appearance of a Heading level
Open the module settings.
Go to the Design Tab and expand the Title Text option group.
Ensure you have selected the correct Heading Level.
Change/customize each of the related options:
Title Font - Allows the choice of a Google font on the respective Heading level.
Title Font Weight - Choose the font weight for the respective Heading level.
Title Font Style - Choose the font style of the respective Heading level.
Title Text Alignment - Choose how the title text will be aligned on the page.
Title Text Color - Choose the respective Heading level's text color.
Title Text Size - Choose the font size for the respective Heading level.
Title Letter Spacing - Choose the space between each letter for the Heading level.
Title Line Height - Choose the line height for the respective Heading level.
Title Text Shadow - Choose the style for text shadow for the respective Heading level.
Customize the appearance of any Heading level for the Entire website
Go to WordPress dashboard → Divi → Theme Customizer.
Go to General Settings → Typography.
Set the Header Text Size.
Set the Header Letter Spacing.
Set the Header Fonts Style.
Set the Header Font.
Set the Header Text Color.
Note: the above options, once set, will be used on all Heading levels, starting with Level 1 (H1) to Level 6 (H6).
Pro tip: the custom settings set for the Header in the Theme Customizer can be overridden on each module level by using the Design Tab > Title Text option group.