Cannot figure out how to get unstuck or start it. Could not see if it was running in the background using ps. (I have Screen shot of App Store OSX Yosemite install with downloaded button depressed but cannot post due to lack of current reputation)
As it turns out, there is a an answer on the Apple support forums. Have downloaded Yosemite but cannot update (Apple.com)
There is an item in the Applications folder called Install OSX Yosemite that can be clicked to start. The other advise is to ignore the estimated time and go away for many minutes.