Limited by Android 8 and higher versions, the phone with Android 8 or higher versions cannot flash the firmware with the lower version (You can check the current version of your phone in Settings>System update).
Your data on your phone won`t be cleared. However, we also recommend you to back up your important data such as contacts, messages before doing that. Moreover, make sure the battery capacity is sufficient (over 30%).
1. Download the corresponding firmware for your phone from official website;
2. Move the package to the root directory of your phone storage or SD card without decompressing;
Note: The root directory is basically the first thing you see when you open a file explorer on your phone. In other words, do not put the file into any folder. The files which you can find in Internal storage interface of Google Files locate the root directory.
3. Install the firmware:
If you can access the phone: Go to Settings>System update>click settings icon at the top-right corner>Local upgrade>click the downloaded package to install it.
If you cannot access the phone: Reboot the phone. After the screen turns black, hold and press Volume Up button and Power button simultaneously until you access FASTBOOT Mode. Select Recovery Mode by pressing Volume Down button, and press Power button to enter it. After accessed Recovery Mode, click Install software>Phone memory>click the downloaded package, tap Install.