Power off the box at the socket.I didn't flash it - bought second hand and this was the image I guess that was flashed to it.
On the setup menu there is a flash image option - when I try to use this - cannot find images please try later. I've unzipped the open atv file and it creates a folder on the usb stick with a name of h8 and files within that folder.
Screenshot
View attachment 86945
Make sure the usb is in the box and power it on while keeping a finger on the standby button and it will flash openatv