iPad unable to upgrade Pandora to Pandora One

When I sign in to iTunes and try to upgrade in Pandora on my iPad first generation, the upgrade button is not highlighted in the Pandora station list, even not respond. Now I can't find a way to upgrade Pandora to Pandora One. But I have set up my account to be able to pay for the service.To upgrade to Pandora one on iOS device, you need to go to Settings > General, and disable "In-app purchases"