Developing apps for this new OS version does not require that you download a new API level in the Momentics IDE for BlackBerry. This release is only for the BlackBerry 10 OS and simulator. Although there are no new APIs in this release, there are some bug fixes that you might want to check out. To see the fixed issues in 10.2.1, visit Fixed in this release.
Before the holidays, we were working hard to get in lots of updates to the documentation. Most importantly, we've added a number of new resources designed specifically to help new users learn the fundamentals about Cascades. Just visit Learning Cascades and walk through the resources we've provided. You'll be an expert in no time.