Mac not updating from 10.8 to 10.10
You need to check inside the featured section of the Mac App Store because new versions of OS X do not always show up in the updates and you have to purchase it (the last three versions have been free) from the Mac App Store and install it. If 10.11 is okay, than that is still available in the App Store. It should be in the featured section or if you search, you will have no trouble finding it. It is only possible to get 10.10 if you purchased it while it is still available, in which case it would be in your purchased tab.