2015 Shield TV, revert from 5.2 to 5.1

A couple of weeks ago, my 2015 shield tv auto updated from 5.1 to 5.2 and while generally ok, one app no longer works. I want to revert to 5.1 to retest the app. Anyone know how to do this ?
Thanks

The easiest way I know of is to use the 5.1 recovery image.

https://developer.nvidia.com/gameworksdownload#?search=SHIELD%20ANDROID%20TV%20Recovery&tx=$additional,shield

Thanks Tim.
Have done that now. all ok.