Open up a terminal and enter ls /dev/ttyACM*.
Connect the UPER board to your linux machine using USB cable.
Type ls /dev/ttyACM* again and you should see two new entries. Most often it will be /dev/ttyACM0 and /dev/ttyACM1. In any case, the device with the lower index should be UPER Command port, and the other - UART bridge (USB COM port adapter).
If you can't see new devices, that means you need to install USB CDC ACM drivers into your system and try again. The installation procedure is distribution dependent, so it will not be covered in here, but if you have trouble doing it - feel free to contact us and we will be more than happy to help.