The Android 4.4.2 KitKat Factory Images have been posted for the Nexus 5, Nexus 7 (2013) and the Nexus 10 devices. Click on the source link below and find the appropriate file and then flash it on your device. Having Factory Image available will allow you to do experiments and customizing your device without having the fear of bricking your device.And once you have successfully flashed your device with the proper Factory Image, Google will ask you to lock your bootloader for some safety reasons.
source: Google