You can alter the contact details listed in the Whois output via your Control Panel.
1) Log into your Plesk Control Panel: https://www.myhostcp.com/cp/
2) Click "All My Domains" (Top Right Corner)
3) Click the "All My Domains" tab.
4) Click on the Domain you wish to edit the Whois records for.
5) Click the "Contacts" tab
6) Click "Edit" at the bottom of the page
7) On this page you can create new contacts, or edit existing ones.
Please Note Once any changes have been saved, please allow up to 24 hours for the WhoIs records to update.