Have you solder the RS485 parts? If yes you can not flash it by using UART (USB to UART).
You must unplug the arduino pro mini to flash it or you can use ISP header to flash it.
I refer in ISP header in satnogs board. If you have a released (with tag v2.0 - v2.2) board, it has ISP header.
If you have board before the release, then you can solder wires in ISP pins of arduino pro mini or
you can unplug it from board and flash it by using an USB to UART converter.