If you want to change the main root FTP Account password, 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.
1- Log into your cPanel account.
2- In the Files section, click on FTP Accounts Icon.
3- Under the FTP Accounts list, click on Change Password or use the Password Generator.
4- Click on the Change Password button.
Now you can access your FTP user account with your new password.