Login to the Plesk control panel (the link and the data for login can be generated from here, clicking on "Active domain management" > "Panel" > "Control panel") and go in the "Email" sections, then click on the email address that you want modify.

(Click on the images to zoom)

For security reason the password must contain at least 8 characters and must be composed by: capital letters, lower cases, numbers and 2 special characters.