iPhoto update seems to be 2G and two attempts to run it failed. It used half my monthly data My iMac is only a few weeks old.  Just after i bought it, it started to automatically run an update for iPhoto.  This went on for several hours and used 2G of data but the update failed.  It tried again with the same result.  I rang Apple and was talked through the process of selecting when I wanted to update and was told that option would show me how big the update file was.  (it doesn't  until it starts to download).  My monthly data of 8G has always been more than I needed, however because of this update failure I used half of it in one day.  Any suggestions on how I can update or why it is failing?  Thanks.
No suggestions as to why it is failing, but I have a suggestion re the update.

Don't. 

No update to iPhoto is going to be urgent. Leave it until it is convenient to take the iMac to somewhere with fast internet access, then hit "Software Update" and let it run its hardest.