To install apps from unknown source on Amazon firestick is very simple, just follow the steps below.
1: Go to the Amazon Firestick Home
2: Click on the Settings icon on the right hand side
3: Select My Fire TV
4: Choose Developer Options
5: Click on Apps from Unknown Sources
6: Click on Turn On
7: Once you have turn it on, now navigate back to the home page and your changes will be automatically save. You can now install the app you wish to install. You can read more on how to install app on fire Amazon firestick on Ecliks