Go to settings, About device, then software update. It will only update if the manufacturer made a 4.2.x update for your device. Not all android devices get updated at the same time as others, or updated at all for that matter.
Search for your device and find out if it is even getting Android's 4.2.x JellyBean update. If it is, just keep checking every day. And make sure it's an OTA Update. OTA means Over The Air, which means your device will update wirelessly over Wi-Fi or data. If it's not OTA, you might need to download software from the manufacturer that will allow you to update through as computer.
