Steps to uninstall SP2 from sql server 2008 R2 X-64 server

Hi,

We have plan to upgrade SQL server 2008 R2 SP1 with SP2.

For rollback plan we need to downgrade back the server with SP1.Can anyone please let me know how to uninstall SP2 from SQL server 2008 R2 server. Do we need to uninstall whole SQL server and install again with SP1 or any other option also there.

Thanks