In cPanel, the document root for any addon domain can be changed directly from the Addon Domains page.
First select Addon Domains from the cPanel main page:
Then Click the edit icon next to the current pathname:
Finally, change the path to any folder within your hosting space:
Once you click the Change button, the server will change where it looks for the files for that site to the folder that you set as the Document Root.
This is particularly useful when dealing with inheritance issues for .htaccess files, where the one for the main domain conflicts with the requirements of domains hosted from folders underneath public_html.
If you have not created the folder and populated it yet, you will receive a 403 error.
Main domains are not modifiable in this manner, they will always default to the public_html folder.