Download Kenbo O21s official stock firmware from the link here, and follow the provided flashing instructions to learn how to flash the stock rom.
If you notice that any of the links provided below is broken or doesn’t work, please post in comments section so we fix and update it ASAP.
File Name: Kenbo_O21S_MT6582.zip
Size: 850 MB
Android Version: 5.0
Firmware – Download