The .apk is already available for download for enthusiasts
Owners of an Android 4.1 Jelly Bean-based smartphone can now download and install the Android 4.2 launcher on their handsets.A leaked Android 4.2 system dump that emerged online a few days ago has already provided a glimpse at the launcher, which is compatible both with stock Android 4.1 devices and with rooted ones.
Apparently, the owners of Nexus smartphones and those who run AOSP /AOKP-based stock Android ROMs will need to root the phone and uninstall the default Android 4.1 launcher. Those with OEM customized ROMs won’t have to do so.
The launcher can be found over at the XDA-Developers forum. Users will need to download it, place it on their phones, use a file explorer to navigate to it, and then install it. Just make sure you read through this forum thread to do things right.