Dude surley you don't expect Asus to support this phone still, it's a 5 year old phone.
Visit asus.com/us/support
Click Zenfone and then you'll see multiple Zenfone. Underneath you'll see a "How to find Model name" . It's important you find out which model number belong to your phone because you have the World Wide ROM, the Chinese ROM and the Japanese ROM. If you're from Europe and you download the Chinese ROM, you'll most likely brick your phone for downloading another regions ROM,
Then click on Driver & Tools and then you'll see Bios and Firmware, click on that and then you'll be able to see a list of firmware updates avaiable for that phone.
Download the firmware onto your PC, ( dont unzip it) plug your phone into your PC and then either drag it into the phones internal storage or micro SD card ( alternatively you can just download it straight on your phone from visting the website on your phone's browser)
Once done. Restart your phone and then once restarted you should get a notifcation appear saying an update has been detected.
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Obviously don't expect Android 8.0 or above for this phone. The last official update (supported by Asus) for this phone is Android M if you want anything higher, you can do but that's up to you. You will end up flashing your phone and unlocking the bootloader and if you do, there's a chance you could end up bricking your phone.... That's entirely down to you
