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.
1. Log into your cPanel account.
2. In the Files section, click on FTP Accounts Icon.
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3. Under the FTP Accounts list, click on Change Password and enter the new FTP Password or generate a new password by clicking on the Password Generator button.
4. Click on the Change Password button.
Now you can access your FTP user account with your new password.