Download iPhoto for Yosemite? How can I download the last iPhoto version for Yosemite?

Looks like any links are invalid now that Photos is out.

And upgrading a pre-Yosemite version does not work. There is an update button but it does not do anything but disable itself when clicked.
Have you purchased the iPhoto? If yes, you should see it in App Store > Purchases tab.

EDIT: The problem was: Clicking on "Update" button didn't update application.

Solution: Delete and redownload application.