1. Login to the plesk control panel
2. go to Home>>Tools & Settings >> Database Servers >> Database server on .MSSQLSERVER2012 >> Change Password
3. Now login to the windows machine via RDP access the Plesk reconfigurator from All Programs >> Parallels Plesk Panel >> Plesk Reconfigurator and choose the "Switch DB Provider"
4. In the dialoge box enter the details as follows
Server Type = MSSQL
Server address = localhost (or .MSSQLSERVER2008 or .MSSQLSERVER2012 or remote)
Administrator login = sa
Administrator password = password you just reset in MSSQL
Database = psa (or any name of database that will be created in MSSQL and will be used by Plesk)
Database user name = psauser (or any name of database user that will be created in MSSQL and will be used by Plesk)
Password = Password123 (password that will be set for database user, and will be used as admin password in Plesk)
5. Than click the "OK" button.