First, log into your Plesk control panel and then click on Domains from the main menu.
Click on the domain that the FTP account belongs to.
Click on Change Hosting Settings.
From here you can now change the password for this domain's FTP account, you can also change the FTP username, should you wish to. Once you have entered the new password, click Ok.
The password change is immediate so don't forget to update your FTP client with the new credentials.