here you go David:
First of all you need to install something called adb (Android Debug Bridge) this is a command-line tool that lets you communicate with a device.
I suspect that, like me, you turned your screen saver to never to stop sleep mode - and I discovered that stops the screensaver but the stick will go to sleep with a black screen after 20/30 mins, not what I wanted!
I think that the issue is that normal apps can’t access system settings, only system apps , so you must use adb, from here: https://www.xda-developers.com/install-adb-windows-macos-linux/
CAUTION FROM HERE AT OWN RISK
this approach does not require you to root the Firestick
remember to unzip the file before running the installer
then read this: https://www.technobuzz.net/how-to-install-adb-and-fastboot-on-windows/
command window, type “adb start-server” (a test)
then follows this guide on how to connect and disconnect to give you confidence: http://www.aftvnews.com/how-to-connect-to-a-fire-tv-or-fire-tv-stick-via-adb/
then for what we want: http://www.aftvnews.com/how-to-set-custom-sleep-or-screensaver-times-on-the-amazon-fire-tv-or-stick-without-root/
good luck, I have done this on 2 generations of Firesticks, wear your badge with pride