Go to Settings on the Fire TV Stick by clicking on the gear icon.
Select My Fire TV.
Select Developer Options.
Click ADB debugging and turn it ON.
Select Install unknown apps.
Select the Find icon from the FireStick home screen then click Next.
Search for the Downloader app then install it.
Click Download.
Launch the Downloader app you just installed and click the URL box.
Enter https://mirrors.kodi.tv/releases/android/arm/kodi-20.0-Nexus-armeabi-v7a.apk?https=1 in the URL box then click Go.
Click Install.
Click Done.
