This is not a simple task I can provide a quick answer to. What you need to do depends on many factors. There is a whole manual dedicated to the upgrade process. You need to spend some time reading the upgrade guide and test it in a test environment.
There have been *a lot* of changes between 10.2 and 11.2 as far as RAC is concerned. I would be surprised if you don't have at least some failed attempts at an upgrade in a test environment before you are happy to go for a production upgrade.