The Max is a different board from the standard player, and its image is not a public download.
Contact Screenly support and we will send you the image and the steps for your device.
We handle it this way because the Max uses Secure Boot, so the device only starts software we signed. Getting the right image onto the right device matters more here than on a board without it.
Note
You should not need this to get a new player working. A Max ships with ScreenlyOS installed and updates itself once it is online.
To return a working device to a clean state rather than replace its image, see Factory reset.