If you have created too many FTP User accounts and some of these have not been used, and you want to remove an FTP Account, then you can accomplish this by following this tutorial.
If you want to increase/decrease the FTP Quota or to use an Unlimited FTP Quota but also want to set a limit, then follow these steps to modify the FTP User quota.
If you want to change the main root FTP Account, then you need to change the password of your cPanel Account. However, if you're going to change the password of a specific FTP Account, you should follow this instruction.
You can use your cPanel username and password to access all folders and files via FTP. However, if you want to create an FTP user account for a specific folder or you don't want to allow access to your root files then you can create a separate FTP Account.