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.