Change FireStick Settings
1. Press the home button on your FireStick remote.
2. Go to the settings gear on the far right.
3. Scroll down and select My Fire TV.
4. Choose Developer Options.
5. Find Apps from Unknown Sources. By default, it’s off. Select it to turn it ON.
Now you can comfortably sideload any app installation file (APK) on the FireStick without any issue.