The firmware upgrade tool usually updates your device to the latest available firmware version. If you require to downgrade your device to a previous firmware there are two methods to achieve this.
Here you can download SG firmwares. Note that these are all available in the SG Firmware Update tool.
Prior to version 1.16.0.b1 the firmware for modules with LoRa functionality was frequency specific. From 1.16.0.b1 and onward, the firmware is region agnostic and this can either be set programatically or via the config block (see here).
1.16.0.b1
As of version 1.12.0.b0 of the firmware update tool, you can now provide a .tar or .tar.gz archive of the firmware you wish to upload to the board.
1.12.0.b0
.tar
.tar.gz
You can also use the CLI version of the update tool to downgrade your device. Will need to get a .tar or .tar.gz archive of the firmware you wish to upload to the board. Then run the following commands:
$ pycom-fwtool-cli -v -p PORT flash -t /path/to/firmware/archive.tar.gz