Hi Suganthi.
You have to perform two steps.
1. First you can migrate the New hardware OS - Windows 2012 with MS Sql 2012 using SAP Homogeneous system copy method.
2. Start the upgrade from SAP ERP6 EHP 7 to EHP 8.
For both you have to refer the SAP System copy guide & Upgrade guide. Refer the SAP link
Regards
SS