OK, I'm getting entirely exasperated here, trying to update. I'm almost resigned to staying on 6.2.18 forever.
I've tried to follow
@ingbrzy in his instructions on the first page: no dice. But then as I noted before, the instructions are completely inadequate.
@adrisouto96 described installing twrp.zip via the updater app in the same way as we used to install weekly ROMs. I tried that. When I reboot I'm back in Mi Recovery.
@Srd971 suggested I rename twrp.zip to update.zip. That did nothing: exactly the same results as above.
Does anyone have an idea of what is missing in these instructions? Because I'm following them to the letter, and they simply don't work.