1. While in the Normal mode, press the SET key to enter the Quick Set Mode. Press the SET key again two more times to display the Rain Increments of Measure.  Press the + button to change the increments of measure to total rain (the rain can only be reset if total rain is showing on the display).
  2. Press SET again to select Total Rain Reset. Press and hold the SET key for three seconds.

The steps are summarized below from the User Manual:

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