To go back to 4.2, it would be easiest to get back to a 507 error (this indicates no OS on the device). You can get there by following these steps. How do I wipe the BlackBerry using Jl Cmder? - BlackBerryFAQ
Download the version of 4.2 for your device. Uninstall the 4.5 version you have installed on your PC. Install The version of 4.2 that you've downloaded. Connect your device and then launch Desktop Manager. Run the Application Loader and install 4.2 on your device.
I hope that helps