Update BOSSA GUI docs

Author: PaulZC

docs/firmware_update.md

@@ -4,13 +4,29 @@ From time to time SparkFun will release new firmware for the LoRaSerial product
 
 ## BOSSA GUI
 
-![SparkFun BOSSA GUI](https://github.com/sparkfun/SparkFun_BOSSA_GUI/raw/main/img/BOSSA_GUI.png)
+![SparkFun BOSSA GUI](img/BOSSA_GUI_Windows.png)
 
-The LoRaSerial utilizes a SAMD21 microcontroller. To update the firmware, obtain the SparkFun BOSSA GUI from [this repo](https://github.com/sparkfun/SparkFun_BOSSA_GUI). The GUI is a Qt5 based python script that can be run from the command line. Alternatively, Windows-compatible executables are available in the [release section](https://github.com/sparkfun/SparkFun_BOSSA_GUI/releases).
+The LoRaSerial utilizes a SAMD21 microcontroller. To update the firmware, obtain the SparkFun BOSSA GUI from [this repo](https://github.com/sparkfun/SparkFun_BOSSA_GUI). The GUI is PyQt5 based on the Python SAM-BA Loader script that can be run from the command line. Alternatively, Windows, macOS and Linux-compatible executables are available in the [release section](https://github.com/sparkfun/SparkFun_BOSSA_GUI/releases). There is a Python package too, which works nicely on Raspberry Pi.
 
 Next, get the latest LoRaSerial Firmware binary located in [LoRaSerial Binaries folder](https://github.com/sparkfun/SparkFun_LoRaSerial/tree/main/Binaries). Note: There are two types of firmware files, one 'with_bootloader' and one without. You can ignore the 'with_bootloader' type file.
 
-![Enlcosure screw](img/SparkFun_LoRaSerial_Enclosure_Screw.jpg)
+Run the GUI.
+
+Click `Browse` and select the firmware file.
+
+Click the `COM Port` dialog box and select the correct port. On macOS, you will see two ports - select the one called `tty.usbmodem`.
+
+Click `Upload Binary` to start the upload.
+
+### Bootloader Mode
+
+The BOSSA GUI will attempt to put the SAMD21 into bootloader mode automatically to perform the update. But we have noticed that this fails occasionally on some machines - it is machine-dependent.
+
+![SparkFun BOSSA GUI](img/BOSSA_GUI_Windows_2.png)
+
+On those machines, you will need to put the SAMD21 into bootloader mode manually by opening the case and giving the reset button a double-tap.
+
+![Enclosure screw](img/SparkFun_LoRaSerial_Enclosure_Screw.jpg)
 
 *Small Philips head screw*
 
@@ -20,4 +36,4 @@ Remove the two side screws and open the LoRaSerial enclosure to expose the reset
 
 *SAMD21 reset button*
 
-Connect the LoRaSerial to the computer over USB. Double-tap the reset button to enter bootloader mode. You should see the blue LED slowly pulse on/off. Now open the BOSSA GUI, select the Binary file you downloaded, select the COM port of the LoRaSerial device, and finally click 'Upload Binary'.
+Connect the LoRaSerial to the computer over USB. Double-tap the reset button to enter bootloader mode. You should see the blue LED slowly pulse on/off. Now open the BOSSA GUI, select the Binary file you downloaded, select the COM port of the LoRaSerial device. ***Uncheck the SAMD21 check-box*** and finally click `Upload Binary`.