The firmware upgrade is tricky on a Tanix Tx6 box. You need to follow the guide exactly to make it work.

1. Download PhoenixSuit Firmware flash tool for Allwinner CPUs. Select the latest version and find the START NOW link. Install it on your PC.
2. Find a suitable firmware file. The links which are made available from Tanix did not work for me, finally I found a working image at GetDroidTips. Ignore the installation instructions on GetDroidTips.
3. Download and unpack the firmware img file, and start PhoenixSuit.
4. Prepare a microSD card (to push the rest button), a USB-A male to USB-A male cable, and the Tx6 box with HDMI connection to a screen.
5. Load the firmware image into PhoenixSuit under Firmware tab. You must do this before connecting the box to the PC.
6. Push the reset button behind the grill next to the microSD card slot. This is best done with a microSD card, it's much more stable than doing it with a paper clip. Do not insert the card into the microSD slot. Note that there is another reset button at the back side of the box, next to the USB ports. Do not use that reset button.
7. While pushing the reset button, connect the USB cable from the PC to the box. If the USB port provides power you don't need to connect the power supply.
8. The upgrade process will start automatically, you don't need to click anything on PhoenixSuit. Wait until the process is finished.
9. Wait for the box to reboot. The first reboot takes much longer, as all settings need to be initialized.