Step 1 - Log into cPanel
Step 2 - The root folder of your primary domain will be the public_html folder. If you click on the File Manager icon in cPanel you can view this folder
Step 3 - Click on the public_html folder on the left. This is your main domain's root folder.
To locate the root folder for subdomains click on the Subdomains icon from the main cPanel page
You can click on the directory to go straight into it's folder in the cPanel File Manager
To change the root folder just click on the pen icon at the edge of the document root
Warning: Please don't do this unless you know 100% what you're doing. This change should only be made by an experienced web developer as if its incorrectly used it can easily break your site and make it inaccessible
Once that's done you will see this pop-up box. Edit the Document Root field and hit the Change button.
Doing the above for addon domains is similar. First click on the Addon Domain icon in cPanel
Here you will see the same options as in the Subdomains screen and you can access or edit the root folder in the same way