To fix the aspect ratio on your Firestick, you'll need to adjust the settings within the device itself. The first step is to access your Firestick's Settings menu by pressing the Home button on your remote control, then selecting “Settings” from the main menu. Once in Settings, navigate to “Display & Sounds” and select “Video Output Format.” Here you will find an option for adjusting both the resolution and aspect ratio of your display output. Depending on which type of television you have connected to your Firestick, it may be necessary to change both settings in order to achieve a proper fit for whatever video content you're trying to watch. When changing these values, keep in mind that higher resolutions require more processing power and can affect streaming performance negatively; so if possible try not raising either setting too high above its current level unless absolutely necessary. After making any changes here, press OK at the bottom right corner of this page before exiting out of menus completely - doing so should save all preferences successfully and allow them take effect immediately!