It is very easy to reset the thermometer. If it happens to be out of alignment (a few degrees off), you can simply turn the thermometer so that it is straight again. Please note: never turn it too hard, or you will break the thermometer. It is better for the thermometer to be slightly loose, but horizontal, than to be too tight.
In this video, we use the 0°C calibration technique (which works on a cold day, as was the case on the day of recording), but you can also use the 100°C calibration technique with boiling water. Insert the thermometer into boiling water, then turn the screw until the pointer reads 100°C. You may need to try this twice in a row to get it to read exactly 100°C.