Like for any new Android version, you need to stay tuned to Google announcements, like the one that Alécio mentioned, as certain APIs could be deprecated/changed (in the case of KitKat, if your application depends on intercepting/reading/storing SMS messages, then you should read this). You also need to figure out whether you want to take advantage of any new API to make your application better in one way or another. It's also possible that a whole new feature could be introduced, one that could be interesting to your application. Just use your common sense.
Finally, to be able to use the latest version, you just need to download the latest SDK and ADT, and increment the targetSDKVersion
number against the latest API level (this is a general good practice). The list of API levels, for each Android version, are available here.