Thank you. This is really helpful. From the forum I have gleaned this advice:
1. Download the official firmware. (Note: Make sure the SKU is matching with your phone.)
2. Place the downloaded file called UL-ASUS_I006D-ASUS-31.1010.0411.122-1.1.25-2208-user.zip (or similar) into the root directory.
3. After a couple of seconds the phone will notify you for a new update.
4. If you can see this notification reboot your device.
5. When you are ready, proceed with the update. It will take about 10-30 minutes to complete.
6. After the update it is recommended to factory reset your device.
Two questions, if you can help further, please:
Question 1 (important)
Step 2 above refers to "the root directory". When I connect ZF8 to PC via cable I don't actually see a drive letter. I see a platform with a "base folder" rather than a "root directory" and this folder contains subfolders named Alarms, Asus, Audiobooks, DCIM, .., .., Ringtones, TwinApps. Is this base folder what is meant by "root directory"? (It is actually the directory /storage/emulated/0/. Using Total Commander or similar I can indeed access the root directory / which itself contains folders data, dev, etc, mnt, .., .., storage, system, vendor; and using TC I could locate the zip file there i.e. at / - I think I could anyway - is this what is intended?)
Question 2 (trivial)
Step 6 recommends a factory reset as a final step. Isn't this essentially what has been achieved already through Steps 1 - 5?)
Thnak you (or anybody else!) for this further illumination.