How to flash Application file using Bootloader to a microcontroller by UART protocol?

1. You get the MCU in bootloader mode (with boot 0 and reset, or however you want)
2. Make sure you have connected your uart adapter to the correct pins (see AN2606 and your device's datasheet/reference manual, I don't remember which one)
3. Make sure you read AN2606 for setting uart parameters in the next step
4. Start stm32cubeprogrammer, select uart, fill the parameters, select the correct COM port
5. click connect

1. You get the MCU in bootloader mode (with boot 0 and reset, or however you want)
2. Make sure you have connected your uart adapter to the correct pins (see AN2606 and your device's datasheet/reference manual, I don't remember which one)
3. Make sure you read AN2606 for setting uart parameters in the next step
4. Start stm32cubeprogrammer, select uart, fill the parameters, select the correct COM port
5. click connect

Get familiar with the protocol defined in AN3155:

https://www.st.com/resource/en/application_note/cd00264342-usart-protocol-used-in-the-stm32-bootloader-stmicroelectronics.pdf

Almost the same question is repeated here.