Designing Board for ARM M0, How to Program
From STM32F030x4,6,8,C Datasheet (page 12)


Boot modes

At startup, the boot pin and boot selector option bit are used to select one of the three boot options:

Boot from User Flash
Boot from System Memory
Boot from embedded SRAM


The boot loader is located in System Memory. It is used to reprogram the Flash memory by using USART on pins PA14/PA15 or PA9/PA10.


From STM32F0zzz Programming Manual (page 44)


Embedded boot loader

The embedded boot loader is located in the System memory, programmed by ST during production. It is used to reprogram the Flash memory using one of the following serial interfaces:


USART on pins PA14/PA15 or PA9/PA10
I2C on pins PB6/PB7 ( STM32F070xx and STM32F030xC devices only)
USB DFU interface (STM32F070xx devices only)

For further details, please refer to AN2606.

And then document AN2606 gives all the details you’ll need (and more).