you can download and install it manually from the forum, or you can just flash the tom manually

You will download the A version. A version is the Chinese version and G version is the Global Version. As your device is already running A version, so please update to stable A version. Also A version is much better than G version in terms of performance. Updates are timely available for A Version. If you have any query regarding this, you can ask here.

Sometimes OTA is not available in the phone. It depends on various factors. There is also other way to update your phone manually. Just download the firmware from the website and you can update your phone by just tapping the firmware file.