In this study, design of a firmware update system for distributed embedded systems consisting of nodes with ARM Cortex M0 microcontrollers is presented. The nodes in the system are connected to each other over high speed CAN Bus 2.0A protocol and the system is controlled by a PC via a graphical user interface. The system allows user to update the Firmware of system nodes through the CAN Bus network. Complete firmware update system for distributed embedded systems with CAN Bus field network is provided by the system designed in this study. With the custom bootloader developed during this study, in application firmware update over CAN Bus feature is integrated to 32 bit STM32F072 microcontroller.