First, there is very little reason to reset the Smart Sensor.
If your sensor is offline, please try the following first: https://support.sengled.com/hc/en-us/articles/360024360114-How-Can-the-Smart-Sensor-be-Reconnected-to-the-Hub-if-it-is-Showing-as-Offline-in-the-Sengled-Home-app-
If you were unable to get the sensor online as explained in the link above or if the sensor failed to add the first time, you would need to reset the sensor to put it into pairing mode and add it back to your hub.
Before resetting the sensor, follow these directions so the sensor is ready to add to your hub: https://support.sengled.com/hc/en-us/articles/360024001674-Adding-a-Sengled-Smart-Window-Door-Sensor-to-your-Sengled-Hub
Reset the sensor when you reach the point in those instructions where it says "At this point, remove the pull tab...".
To reset the sensor, use the provided tool, or something equally thin like a paper clip, to press and hold the reset button inside the unit for eight seconds. If done successfully, a red light on the face of the sensor will flash twice.
After the sensor is reset, you will have one minute to add it to your hub.