The CAM-L21 has several variants. Using the wrong firmware will cause the update to fail (or worse). Check your phone’s box, remove the battery (if removable), or look in Settings > About Phone .
If you search for this firmware, ensure the file is specifically for CAM-L21 (not CAM-L03 or CAM-L23). Using the wrong UPDATE.APP from an SD card will permanently hard-brick the phone because Huawei's dload method overwrites the bootloader. cam-l21 firmware sd card
Back up all personal data, as this process may wipe the internal storage. The CAM-L21 has several variants
Firmware updates can bring a range of benefits, including: remove the battery (if removable)