Using the latest version of Keynote 6.1 has the following conditions:
it only runs in Mavericks, it wont run on an earlier OS
it is not an update but a new application therefore it is a paid upgrade from the Mac App Store
it is available as a free download if you purchased a Mac from October 2013 onwards, if your Mac is older it is a paid for download
If iWork 09 is installed when you obtain Keynote 6.1 the older applications are still installed allowing the two versions to be used for the diffent file types used. Best practice currently is to use 5.3 to oen those files and 6.1 to create new files.