Where can I download/update to OS 10.10?
I don't think Yosemite is still available for download, but El Capitan is. El Capitan is basically just Yosemite with some bug fixes, and has the same system requirements as Yosemite, so there's no real reason not to go to El Cap if you're already planning to go to Yosemite. (High Sierra is, obviously, even better if your Mac meets the requirements.)