Hi
MERQRThank you for your asking. You may refer to
AsusYouzr's reply, which is a pretty solid answer.
Please note that our official website currently provides WW-17.2017.2008.432 & WW-17.2017.2012.438 image file to avoid any misinstallation.
ZenFone Max Pro (ZB602KL) - Support#Android 10 to Android 9 Guide
[Phone] How to downgrade OS version from Android 10 to Android 9 target version? | Official Support ...**Please backup the device before practicing the process.
1. Download "MaxProM1_downgrade_to_9" and then put the image file into the root directory of the MicroSD card.
2. Insert MicroSD card into phone and then power off the device.
3. Long press with "Power" key and "Volume Down" key together and stop after feel the vibration to enter Recovery Mode
4. Use "Volume Down/Up" key and press "Power" key to choose "Apply update from external storage"
5. Select the downgrade file
6. When the updating process is done, Choose "Wipe data/factory reset" and tap"Yes" to perform the master clear.
7. "Wipe cache partition" > "Yes"
8. "Reboot system now"