Can't update from 10.10.5 to 10.11

My poor old machine (MacBookAir) is still running Yosimite 10.10.5. I worked and worked to get enough memory free so it could at least update to El Capitan. I downloaded the installer and it said it went through the whole process successfully to install El Capital. I restarted the machine. When I go to "About This Mac" it still says it's running Yosimite 10.10.5. WHY. Where did the update go? Why won't it run? I've been trying to fix this for a couple years now and it got me at my wits end.