There are dozens of MXQ S805 board variations (HD18Q, MXQ_S85, etc.). Using the wrong firmware can "brick" your device (make it unbootable). Always back up your current firmware using TWRP Recovery before attempting an upgrade. to ensure you pick the right firmware?
: You need a Windows PC, a high-quality USB-A to USB-A (male-to-male) cable, or a standard SD card. Mxq Amlogic S805 Android 5.1.1 Lollipop Download
What is the of your MXQ box motherboard (e.g., blue, black, or red)? There are dozens of MXQ S805 board variations