First, log into your Plesk control panel and then click on Domains from the main menu.
Click on Domain that you wish to modify.
Click Websites & Domains from the tab menu at the top of the page.
Click on Show more to see additional configuration options.
Click on Password-Protected Directories option.
Click on Directory you wish to change the password for user.
Click on User that needs password changed.
Enter a new password. Click on OK to accept change.