You do not have to "cancel" the previous download. If you have the most current version of iTunes loaded on your computer, and you have the same AppleID that you purchased the Keynote app with signed on to iTunes, and you have the computer authorized for that AppleID, and you have iCloud your iCloud account set up on the computer, then you can (in iTunes):
Click on Store
Click on Purchased under the Quick Links section in the right-hand sidebar
Click on the Apps icon in the Purchased screen
The Keynote app should appear with a little cloud next to it. Click on that and download to your computer.