Upgrade ROM for an already unlocked FireTv Stick 4K

Connect to the FireTV via adb over Wifi

adb connect <FireTvipAddress>

Push file to the FireTV

adb push romFile.zip /sdcard/romFile.zip

Create Update script for TWRP

adb shell
su
echo "--update_package=/sdcard/romFile.zip" > /cache/recovery/command
echo "--wipe_cache" >> /cache/recovery/command
reboot recovery

After upgrade, disable OTA Update

adb shell
su
pm disable com.amazon.tv.forcedotaupdater.v2
pm disable com.amazon.device.software.ota
pm disable com.amazon.device.software.ota.override