Inside your FTP Manager in CPanel, click "FTP Accounts". Now, simply create an FTP account with the same name as your already existing subdomain.

Example: If your subdomain is "abc.yourdomain.com," create an account with the username "abc."

Enter the new details into your FTP program on your local computer as follows:
    FTP Address: ftp.yourdomain.com
    Username/Login: abc
    Password: The password you've chosen for this account.