1. Open regedit
  2. Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL10.SQLEXPRESS\MSSQLServer
    or
    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL10.MSSQLServer\MSSQLServer

  3. Change LoginMode from 1 to 2
  4. Restart the SQL Express service
  5. Open SQL Management Studio
  6. Login using Windows Authentication
  7. Go to the Security, Logins users and select properties on sa user (which is disabled by default when Windows Authentication mode was selected on install)

  8. Enable the sa user, and set a new password


  9. Click OK to save sa user details
  10. Close SQL Management Studio
  11. Open SQL Management Studio, login to SQL Express using SQL Authentication with sa user, and password you set.