The BT121 has three wired update methods.
Here is an overview. Detailed instructions can be found on the Silicon Labs community site.

• Method 1 - BGAPI-based DFU over UART

BT121's firmware "iWRAP" provides API commands for DFU (Device Firmware Upgrade). You can update using this API command and UART. Only the firmware can be rewritten, and the Bootloader cannot be rewritten.
If you are designing firmware using BGScript, you will need to implement a mechanism that triggers the transition to DFU mode. The DFU function is implemented in the sample code (bgdemo.bgs) in \example\bgdemo of the SDK, so you can refer to it.

• Method 2 - ST-based DFU over UART

It is a method that uses UART and BOOT0 pin to rewrite not only the firmware but also the entire Bootloader. Unlike Method 1, which can be easily rewritten with API commands, it is necessary to implement a sequence for rewriting. A sample code is available.
When rewriting in the manufacturing process, many customers use Method 2, which allows rewriting of the Bootloader.

 

• Method 3 -Re-programming over the ARM Serial Wire Debug interface

This is a method of rewriting using ARM's debug interface instead of UART.