To rename your domain name please follow the followinig steps:-
1) Login to your Plesk control panel.
2) Click the "Domains" shortcut in the navigation panel.
3) Click the required domain name in the list.
4) Click on "Domain administration" and then click on "Rename domain"
5) Enter the name which you want to rename for your current domain.
Make sure you click on "OK" once you have made the required changes.