Cancel Installation of El Capitan

I have tried to upgrade to El Capitan in the past but it doesn't work on my iMac (Late 2012 / 2.7 GHz Intel Core i5). I had to use Time Machine to do a system restore back to OS 10.9.5. While trying to disable the annoying update notifications I accidentally initiated the update again. The El Capitan files are downloaded (probably from my previous attempt) so I'm afraid if I restart it will start installing again. How can I disable the update? Thank you.