Select the text to be made into a title.
In the Common
tab, click on the Heading Style 
button.
From the sub menu which appears, select the desired Heading Style.
This command is used to assign H1, H2, H3, H4, H5 or H6 heading styles to text. The first level (H1) formats text in a larger font and/or darker than normal text, while the sixth heading type (H6) displays it in smaller. With any of these tags, the titles or headings are automatically placed on a line of their own within the document.
Select the text to be made into a title.
In the Common
tab, click on the Heading Style
button.
From the sub menu which appears, select the desired Heading Style.
You can use the shortcuts SHIFT+CTRL+1, SHIFT+CTRL+2, SHIFT+CTRL+3, … the number corresponds to the heading style level that you want to apply (see list of shortcuts).
The selected text will be surrounded by the <h1> and </h1> tags to the <h6> and </h6>.