Yes, you can change your cPanel account username. This feature is not found in your cPanel because cPanel accounts are tied to the username, your home directory, configurations, email, etc. 
Please contact us for assistance by support ticket or live chat if you would like to change your username. 

    However please notice that your username can not be more then 8 characters length.

And also we should notify you about possible issues which may occur after such a change:
- If you have a web site which is using a database, the site's MySQL configuration should be changed accordingly
to the new account user name. Usually it is done by editing the configuration file;
- The new cPanel username should be the same length or shorter in order to prevent MySQL from truncating the
account's database names and database usernames;
- Also an email account which has the same user name as cPanel has cannot be created.

To request changing your cPanel username, please, submit a ticket to our Hosting - Support department. Please indicate the following information in the ticket: 

   Main domain name: 
   Namecheap account username: 
   cPanel username:
   New cPanel username: