Amazon Fire TV Stick has an app market from which you can download your desired app. If you decide to download an app from outside the market, then you need to disable a security measure on your Fire Stick which disallows installation of apps from outside of the Fire Stick approved apps market.

Simply follow below instructions to disable this feature:

1-From home screen, click on Settings

2-Now choose Device

3-Go down and choose Developer Options

4-Click on Apps From Unknown Sources

5-From the shown pop up, click Turn On

For a more detailed tutorial, click here.