1- Go to Settings ⇒ My FIRE TV ⇒ Developer Options. Make sure Apps from Unknown Sources are Turned ON.
2- Open Downloader App and type the following url:
And Click on the "Go" Button.
3- Follow the on-screen instructions. After download finishes, you will be able to find the .APK file in the Downloader app. Click on it to install 9Xtream.