In the back side of my router, I can see the model no is TL-WR840N(EU) Ver 6.1 and in my router page the hardware version is: TL-WR840N v6 00000006

In the download center the firmware upgrade is only available for v6 and v6.20 but not for v6.1 

If I want to upgrade my router's firmware which version should I choose v6 or v6.20?