To enable "Apps from Unknown Sources" on your Fire Stick, navigate to Settings - My Fire TV - Developer options and toggle the switch to On for "Apps from Unknown Sources", then confirm the warning. If you don't see Developer Options, go to Settings - My Fire TV - About and repeatedly press the center button on your Fire Stick remote on your device name until you see a message that you are a developer.
Step-by-step Guide
1. Go to Settings: From the Fire Stick home screen, scroll to the top and select Settings.
2. Select My Fire TV: In the Settings menu, select My Fire TV (or Device on some models).
3. Enable Developer Options (if hidden): If you don't see Developer options, scroll down and select About. Then, highlight your Fire Stick's name and press the OK button on your remote repeatedly, about seven times, until you see a message saying you are now a developer.
4. Select Developer Options: Go back to the My Fire TV menu and select Developer options.
5. Toggle Apps from Unknown Sources: Locate Apps from Unknown Sources and toggle the switch to the On position.
6. Confirm: A warning message will appear. Select Turn On to confirm your decision.
You can now install apps from outside the Amazon Appstore using apps like Downloader.