I assume you don’t have MQTT access to your device, as you want to change the MQTT setting. Is that correct? Otherwise you could change the MQTT setting through MQTT system command, or alternatively completely resetting your ESP32.
Other than the above I don’t think it’s possible to get to the set up page again once the details have been saved to the device’s flash memory., as described here
Note that the web portal appears only on first boot, if you want to configure again the setting you can do a long press on TRIGGER_GPIO or erase the settings.