Are you trying to downgrade to Android 9? Is your phone ZE620KL?
If yes, you can find the downgrade file in the link below under "Version Android 10 downgrade to P".ZenFone 5 (ZE620KL) BIOS & FIRMWARE | Phone | ASUS Global
Please follow the steps below and make sure to backup data before downgrade.
Steps of Update:
1. Prepare a MicroSD card.
2. Check software version of your device**
3. Only apply to same SKU update, Example: WW->WW, CN->CN, TW->TW, ID->ID.
4. Software update cannot transfer the software SKU.
5. Wrong SKU may cause update failure, please update the same SKU version only.
6. How to manual update firmware please refer to[Phone] How to downgrade OS version from Android 10 to Android 9 target version? | Official Support ...
Thank you. 😊