JUST follow the simple instruction CAREFULLY specially STEP 2:
Step 1) Download Android 9 Firmware file from asus website.
Step 2) Rename the downloaded file to the name highlighted in bold UL-ASUS_I001_1-ASUS-17.0210.2001.65-1.1.97-user.zip
Step 3) just put this file in the internal memory as u did while upgrading to android 10. Restart and mobile will detect this file and it will get installed.
Step4) Thats It !!!!