Which password is that? The Plesk login password? The FTP password? The password of a CMS that is installed to manage your website? A password of a user account of the website itself?
You can either reset the Wordpress password by using the "Setup" link in the Wordpress toolkit. When you set new access credentials there, they will be set in Plesk for the Wordpress "login" button, but also in Wordpress for the regular wp-login.php login.
Or you can edit the password in the Wordpress users table in the database directly. There you can set a new password by choosing the "MD5" field conversion algorithm on the plain text entry that you do, e.g. in phpMyAdmin.
All kinds of options, the above mentioned included, are also explained here: Reset Your Password