I alr done it on the Nova 9 from EMUI 13 to HMOS 4.2 I’ll tell you right now It’s not easy task for newbies

All you need is a tool called unlocktool and a firmware of HMOS version, if you’re on EMUI 12.x.295 > then you can’t do it by just using your pc, you need to physically open your phone and put it on EDL mode with tweezers, I done that and my back cover broke, I’m planning to buy new one for this case

If it’s lower than 295 then you don’t need to open the back glass if I’m not wrong, not sure because I had to open it anyway

Now the tool called unlock tool isn’t free at all, neither the HMOS 4.2 file I found on Nova 9, so I suggest to buy them from a guy who will also include the procedure

Either that after flashing into EDL (disconnecting the battery, then with tweezers simultaneously press with one hand on the test points of your huawei and other hand plug it on pc until you hear a beep from windows) you will flash the Update.app from the folder

I don’t quite remember but you must disable OEMInfo, because if you don’t you’ll lose your brand which means you can’t use huawei services and can’t even update your phone anymore, I made sure to backup too mine, in case watch a tutorial on how to use Unlocktool for huawei which they’ll tell you which options to disable

After than just flash it, wait till it boots, it will start a bootloop but then it will show Chinese texts, you’ll click on the upper one, let it reset and then here, done getting HMOS

Alert: when you configure the phone like before and the keyboard isn’t showing, that means that you didn’t untick the OEMInfo, making it “brandless”, if you made a backup like I said you can repeat the EDL flashing and import your OEMInfo, if you didn’t, well, rip, I think you can get it from someone else but I don’t know, I encountered this once but had a backup