If the OTA update isn't showing up yet, or if you are stuck on an older version, you can use the HiSuite application on a Windows PC. Connect your phone via USB, and the software will check the server for the latest authorized firmware for your specific region (CUST).
Any website claiming "EMUI 12 for Y9 Prime 2019" is either: firmware huawei y9 prime 2019 stk-lx3 emui 12
Do not search for "EMUI 12 firmware" for STK-LX3 – you will not find a legitimate Huawei package. Instead, to restore the latest official EMUI 10, or check forums like XDA Developers for custom ROMs if you truly want a newer Android look (though that requires an unlocked bootloader, which Huawei no longer allows easily). If the OTA update isn't showing up yet,
Here is the explanation and what you should look for instead: Instead, to restore the latest official EMUI 10,