Installing the wrong firmware can "brick" your unit (render it unbootable). Before searching for a download, go to Settings > About Device (or System Information) and write down the following:
Usually Rockchip (PX3, PX5, PX6) or Allwinner (T3, T8). amedia auto 1024x600 o update
Ensure the USB is FAT32. Some units only recognize the rear USB port for updates, not the front one. Installing the wrong firmware can "brick" your unit