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
Password: The password you've chosen for this account.