1. Check that the HDMI cable is securely connected to both the TV and external device.
2. Check that the TV input is correctly set to the HDMI port that the external device is connected to.
3. Change the resolution on the external device settings menu.
4. Set the picture size manually on the TV to the desired resolution.
5. Try achieving the correct resolution using the Auto Picture Size feature on the TV.
6. Try resetting the TV to its factory default settings.