Hardware

There are the pins C2 & C2CK on the board. With a Arduino you can build a programmer to read/erase and program the flash.
Software for the Arduino: Program EFM8 devices using an arduino

• C2 Interface
  Use this header to flash a new firmware for the EFM8BB1.

• RS232
  Use this RS232 connection for debug/testing. Put the switch S2 to OFF when connecting.
  For normal operation the switch S2 have to be in ON position.

If you are running Sonoff Tasmota the firmware can be easily upgraded with a small hardware hack:
Rf chip firmware upgrade

Sonoff RF Bridge v1.0:

Sonoff RF Bridge R2 v1.0:

The main difference of the boards are the transmit part.
On the R2 it should have an increased transmit range.