Download Android 9 v2.1.9 stock firmware for Sunvell Q Plus TV Box from here or here. Download Android Oreo 8.1 stock firmware from here. Flash it from PC using LiveSuit v1.11 or from SD card using PhoenixCard v3.09.
Model: Sunvell Q Plus TV Box
Image file size: 824 MB
Platform: AllWinner H6
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.