Upgrading iMac with OS YOSEMITE 10.10.5

My parents have a 2011 iMac that is running on Yosemite 10.10.5.

We need to update the OS because we need a more up to date version of itunes to update their iphones in order to get a check in app on their phone.

What OS should we update the iMac to?

I have the pleasure of having to assist my parents do this remotely, any help would be appreciated.

UPDATE - Looks like i need High Sierra however when we try and download it from the app store we are getting an error message '"This copy of the install is damaged and can't be used"