It is not possible to roll back the firmware on the UX2 after it has been updated.
Make sure that within POD Farm that you have the correct input source select for your tone. To do so, click on the mixer view button located directly to the right of the Line 6 logo that is in the top center of the application. From the mixer page make sure that above Tone A and Tone B where it says "Input" that it is set to instrument if you are trying to test your guitar.
If it is set to instrument and you still can't receive any input, make sure to try removing any additional unnecessary USB and firmware devices from your computer when troubleshooting this issue. Make sure to connect it directly to a USB port and not a USB hub, extension, or keyboard of any kind, preferably connect it to a USB port on the rear of computer. Try testing it using additional USB cables and every USB port available.