You cannot upgrade a 4.2.x.x device over the air.
Wireless Update requires 4.5.x.x or higher on the handheld.
Additionally, the handheld must be running a carrier-approved version of the handheld OS and must also offer an OTA version of the updated OS.
You can read more about OTASL here:
OTASL - Official BES Provisioned Versions - Port3101.org : Your BES Connection