Check your PCB (Printed Circuit Board) before searching for the download. Open the box (carefully pop off the rubber feet and unscrew the screws). Look for a silkscreened text:
When downloading and installing firmware updates for your MXQ Amlogic S805 TV box, make sure to: mxq amlogic s805 android 511 lollipop hot download
This is a database for Android ROMs.