Headings can be found under the format dropdown menu. The available options are heading 2 - heading 6.
To add a heading, select the text you want to be a header, and choose the heading size from the format dropdown menu (next to the styles dropdown menu).
Headings should show a hierarchy of content starting with heading 2 and going down.
For example, a section of content should start with heading 2 and supporting content within that section should start with a heading 3 and content under the heading 3 should start with a heading 4.
For more information visit: