How do I upgrade from OS X Mavericks to Sierra

I have a 27-inch, Late 2009 imac with a 3.06 GHZ Intel Core 2 Duo and currently running OS X 10.9.5 (13F1911) and I need to update the OS so that I can update my itunes so it will be compatible with the new iphone X.

I tried to see if OS El Capitan would be compatible but apparently it's not according to the app store function when I tried to download it stated it was incompatible. I haven't had much luck finding OS X Yosemite in the app store to try and see if it's compatible.

I'm looking for help locating the actual download link or recommendations.

Thanks for your help!