You can access it from the STM32 - it is ICE40_CRST_PIN.
Why would you want to access it from the ice40? It would cause you to lose your configuration. What do you want it to do?
If you want an external pin to reset FPGA logic, you should be able to use a button. But the buttons seem too glitchy for that. I have tried using them and just get random resets. So I use another Pmod pin. (I also modified the firmware to be able to use !CS to cause a reset when something connects to USB).
If you want to use reset to cause a load of a configuration from flash memory, you need to use SB_WARMBOOT for that.