Firmware update mechanisms are essential for maintaining the security, reliability, and functionality of embedded systems during their operational lifetime. However, Designing a secure Firmware-Over-The-Air (FOTA) solutions in resource-constrained microcontroller-based platforms includes significant challenges, including security against firmware tampering, securing of rollback attacks, preventing leakage of cryptographic keys, and maintaining system robustness in the presence of communication failures or power interruptions. This thesis presents the design and implementation of a secure and resilient Firmware-update-over-the-air (FOTA) framework for STM32-based embedded systems. The proposed approach follows a threat-driven and design-oriented methodology in which security requirements are derived from a structured threat model and integrated into the system architecture.