Follow the below simple procedures to accomplish the same.
For WHM users who is having root access to the server will be able to do the same using a tool provided by the cpanel. You will be able to access it from your WHM interface once you have logged in as a root user. You can access the tool from
Home >> Transfers >> Convert Addon Domain to Account
Once you are in the "Convert Addon Domain to Account" page, you can see all the addon domains. You can also search the addon domain using the domain name or the username.
Find the addon domain you need to convert to separate account and click on "convert"
Once you are in the new window, populate the details like username, contact mail, package information etc.
Click on "Start Conversion" at the bottom of the page.