What you received from ThingsBoard is a message with an attributes update. Then you need to check if it's a new version and request new firmware from the server.

See this documentation for more information:
https://thingsboard.io/docs/reference/mqtt-api/#firmware-api

Also, there is a ThingsBoard Client SDK. It's a wrapper on the MQTT protocol. It's available in Arduino IDE.
You can see OTA update implementation here OTA_Firmware_Update.h or how to use this library here.