Hello,
Use Windows Update to apply SP2 or later to SQL Server 2008 first. SP2 for SQL Server 2008 is required to upgrade SQL Server 2012.
Now please download SQL Server 2012 Express, run SQL Server setup, choose Installation on left side of the SQL Server Installation Center, then click on
"Upgrade from SQL Server 2005, SQL Server 2008 or SQL Server 2008 R2". This is called an in-place upgrade.
Another option is a side-by-side upgrade. Install a new instance of SQL Server 2012, and then backup databases on the old instance, and then restore the database
on the new instance. You can also use detach/attach.
Hope this helps.
Regards,
Alberto Morillo
SQLCoffee.com