Leave your PCB version and Wi-Fi chip in the comments below for a custom firmware link.
A Russian forum (requires translation) that houses the most extensive archive of S805 firmware. EntertainmentBox (EBox): mxq s805 firmware 51 download link
(18+), Android 5.1 may still feel sluggish or lack support. Many users prefer: Leave your PCB version and Wi-Fi chip in
Unzip your downloaded firmware files. You should typically see files like factory_update_param.aml , recovery.img , and a .zip or .img firmware file. Enter Recovery Mode: Insert the SD card/USB into the box. Unplug the power. mxq s805 firmware 51 download link
via a PC or by placing the files on a FAT32-formatted SD card and using the "Toothpick Method"