ecobee devices will update themselves automatically, provided they are connected to Wi-Fi. When a firmware version is ready for the public, it will be pushed out gradually over a period of time to all applicable ecobee devices.
You can find the firmware version of your ecobee thermostat by going to MAIN MENU > ABOUT, through the thermostat interface.
Auto-Upgrade Firmware Versions as of January 19th, 2020 :
|ecobee SmartThermostat with Voice Control||18.104.22.168|