Hi,
You will have to go in solution manager and then generate the xml file .
During the generation of the stack file you need to select EHP6 and then generate the stack.
Once the stack is generated download the patches and the SUM tool .
Extract the SUM tool and run the upgrade.
Just a demo of sum tool.
http://scn.sap.com/docs/DOC-25113
Just check the guide for the minimum sp required to upgrade to EHP6...
Thanks
Rishi Abrol