Hello,
Please check the information available on Working with headings using PHP methods and HTML contents that details how to accomplish this task.
Please note that if you are using a custom template, then your heading style names may not be the same. As explained on the previous page:
The base phpdocx template doesn't include the Heading 1 or Heading 2 styles but some other ones called Heading1PHPDOCX, Heading2PHPDOCX...
Using a custom DOCX template, you can get the available style names using Indexer, getWordStyles or parseStyles.
Please also note, that as your license includes LUS, if you send to contact[at]phpdocx.com the DOCX template you are using we'll check it and generate a custom sample script.
Regards.