Firmware for the A5X PLUS Mini android TV firmware is available for download. This is latest firmware that is based on android 7.1 nougat. To install it, you will have to download Rockchip drivers and RK batch tool. For upgrading, any windows PC with 7, 8 or 10 will work. You will also a microUSB cable to connect your android TV to the computer. Follow these steps.
Download RK driver assistant from here. Start it and click on install driver. If you get a security message, click on Install button to proceed and it will install the driver needed to connect your A5X Plus mini android TV to the computer. Close it and disconnect your android TV.
Download and start RKBatchTool. Download the official 7.1 firmware from here. Click on firmware button in RKbatchtool. Select the file bearing .img extention. Now hold the reset button of A5X PLUS Mini and connect it to the computer. Click on restore button and it will start downloading the firmware to the android TV.
It will take a while to install the firmware as a lot of files are overwritten. It will also delete any data stored on it so make sure you back the data up before doing any kind of upgrade.