IntroductionOTA (Over the Air) update is the process of loading the firmware to ESP module using Wi-Fi connection rather that a serial port. Such functionality became extremely useful in case of limited or no physical access to the module.OTA may be done using:Arduino IDEWeb BrowserHTTP ServerArduino IDE option is intended primarily for software development phase. The two other options would be more useful after deployment, to provide module with application updates manually with a web browser or automatically using a http server.In any case first firmware upload have to be done over a serial port. If OTA routines are correctly implemented in a sketch, then all subsequent uploads may be done over the air.There is no imposed security on OTA process from being hacked. It is up to developer to ensure that updates are allowed only from legitimate / trusted source. Once update is complete, module restarts and new code is executed. Developer should ensure that application running on module is shut down and restarted in a safe manner. Chapters below provide additional information regarding security and safety of OTA process.

Source: OTA Update · ESP8266 Arduino Core