There is no specific way of resetting memory. Once you have taken 99 temperature readings (it will appear as M99 on the bottom right corner of your screen), the most recent temperature reading taken will start to overwrite the oldest temperature memory setting (M1). This will then continue, overwriting the next memory slots in order (M2, M3, M4, etc.).
The TMP101 Infrared Thermometer (left) and the TMP102 Infrared Thermometer (right) both have 99 available memory slots.
For more information, refer to the linked video below: