Download Android 7.1 stock firmware for Magicsee C400 Plus TV Box from here, here and here. Download previous firmware from here. Download Amlogic USB Burning Tool v2.1.6 from here or download Amlogic Burn_Card_Maker v2.0.2 from here.
Archive: c400 plus s912 20180627 aml_upgrade_package
Model: Magicsee C400 Plus TV Box
Image file size: 795 MB
Platform: Amlogic S912
Image: c400 plus s912 20180627 aml_upgrade_package.img
Build number: q201-user 7.1.2 NHG47L 20180627 release-keys
Caution: Please do not update firmware unless you know what you are doing, you really want it or it is absolutely necessary. Firmware updates may occasionally have bad results, like bricking your device, temporarily or permanently. The firmware update process is done entirely at your own risk. Before attempting to update the firmware, back up your data.