If you would like to alter main root FTP Account then you need to alter the password of your cPanel Account.
However, if you would like to change the password of a specific FTP Account, you can follow the subsequent instructions:

1. Log into your cPanel account.
2. In the “Files” section, click on “FTP Accounts” Icon.

3. In the “FTP Accounts” list, press on “Change Password” or else you can also use the “Password Generator” option.

4. Click on the “Change Password” button.

Now you can easily operate your FTP user account with your new password