La Crosse Technology WS-9121 - Manual (Page 27)

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RESETTING THE MINIMUM AND MAXIMUM READINGS:
User may reset the minimum and maximum temperature data to the
current value by the following steps:
1.
Press the “MIN/MAX/+” key once to display the minimum data.
2. Press and hold the “MIN/MAX/+” key for about 3 seconds to reset
all the minimum / maximum data to the current values in a single
action.
3. Data of all outdoor and indoor sensor will be reset at the same
time.