Hi! You could consider the following 2 ways:
1.) Change from an Arduino Uno to an Arduino Leonardo (or similar, based on a Atmega32u4). This chip has a build-in usb-interface.
2.) If for some reason, an Arduino Uno is necessary, then you can communicate over UART. There you have to consider the following points:
(I have drawn you a schematic, but I don't know how to upload pictures in this new forum, sorry )
Different IO-Level: You need a bidirectional level-converter for all of the following lines (you can create one with a n-mosfet, and two pullup-resistors per line):
Radxa_UART0_RX <--- converter ---> Arduino_TX
Radxa_UART0_TX <--- converter ---> Arduino_RX
Radxa_UART0_RTS <--- converter --- || ----> Reset (100nF capacitor, converts the RTS signal in a short Reset-pulse to enter arduino bootloader, only needed if you want to programm arduino over UART)
Radxa_GND <------> Arduino_GND (no converter needed )
Further, I would recommend you to add some protection diodes at the Radxa_UART0_RTS signal (I think, you go best with two schottky-diodes)
To make long things short: I recommend you to take an Arduino based on a Atmega32u4 chip.
Otherwise, if you have questions to the UART-way, please let me know, or if you know how to post pictures, I can add the schematic here.
BTW: Some time ago, I made an Arduino-shield for the radxa, have a look at the schematic, if you want
(2 Remarks to the schematic:
I had no protection diodes, and i fried the pin. So I highly advise some pin protection. I dont have level converter between Arduino and Radxa as explained above, because the Arduino on the shield is running at the same IO-Level as the Radxa)