How to access BIGTREETECH firmware
The previous answer is good, but here's one specific for Bigtreetech. 


Install platform.io. I use the command-line interface (CLI)
Modify your marlin files. You can clone existiing firmware for your board from the BigtreeTech Github for your board. 
Remove microUSB card from your Bigtree tech board
Plug microUSB card into microUSB reader, and the microusb reader into the computer. You should be able to read your microUSB card
Enter the command in the root directory of your Marlin files: platformio run -e STM32F103RC_bigtree_USB. At least, this is the one for my board. You should have to run this in one folder before the Marlin folder. 
It creates a file called firmware.bin in the directory .pio/build/STM32F103RC_bigtree_USB/firmware.bin. Copy it to your microUSB card, replacing and deleting any existing .bin. You can use the name firmware.bin.
Remove microSD card from computer and plug into board.


In your Marlin Configuration.h file, there should be a variable called something like MACHINE_NAME. If you make that name custom, then it will appear in the Octoprint terminal when connecting to your board, letting you know that you have updated the firmware.