Do not attempt to downgrade from Android 6.0 (if you previously updated) to Android 5.1 using this method. Downgrading without a full factory wipe often causes IMEI (Network) loss.
Check that the folder is named exactly dload (lowercase) and contains only the UPDATE.APP file. huawei cun-l21 flash file sd card
(Long-form, comprehensive, and ready for publication) Do not attempt to downgrade from Android 6