1) Upload ROM file to SD card
2) Enter to TWRP
3) Wipe > Advanced Wipe
Dalvik / ART Cache
System
Data
Internal Storage
Cache
Vendor
4) Swipe to Wipe
5) Install > Choose ROM file
6) Wipe > Format Data > Yes
7) Reboot
If the smartphone does not start for a long time, hold down the power button until the smartphone restarts. After that, it should start
For Redmi 6A: MIUI_12.0.2.0-cactus.zip
For Redmi 6: MIUI_12.0.2.0-cereus.zip