OS update for Mid 2011 IMac
Hi - I have a mid 2011 Imac that is running Yosemite. Been using it primariy as a second screen but would like to start using it as an additional computer again. Would like to secure it as best as possible. It seems that the most advanced OS update I can put on it is High Sierra. Is that the case and if so should I upgrade to it and how can I do so? When trying to download the update on that computer from the App store it says the copy is damaged and won't download.
Thanks!