Huawei has already released the new Emui 10 firmware with Android 10 to its Mate 20 series smartphones by now.
However, you might not be able to find it under the normal Settings > System & updates > Software update section of EMUI 9.1.
You will need to ensure your Huawei Mate 20 series handphone has been updated to the latest version of EMUI 9.1 (Android Pie). Then, you can proceed to search for EMUI 10 by using the HiCare app and perform the firmware upgrade from EMUI 9.1 to EMUI 10.
After you have open the HiCare app in your phone, make sure it has the latest version first. Go to Me > Settings > Check for updates. Your HiCare version should be 10.0.5.341 or above.
Then, go to Services > Update > Check for updates. You should be able to find EMUI 10 and perform the upgrade there.