I just updated to El Capitan. The installation seems to have gone through with no problems, and the operating system works well. However, under the App Store tab "purchases", El Capitan is shown as "An error occurred" in red, and the clickable icon offer the option "Download". Should I download it again and re-install?
It might not have actually installed. From the Apple menu, choose About This Mac and verify that you are running El Capitan. If you still have the installer in your applications folder then this too might suggest that it did not install. If it didn't then I would delete the installer from Applications and download El Capitan from App Store again and preferably perform a clean install although not required.