We need more information than that.
What time zone? UTC+02:00?
How did you install? Using setup.py, or Debian? Or, something else?
Do you want to zero out all rain data for the day? Or, is there just one bad value? If just one value, roughly how big is the value?
Try this and tell me what you get
echo "SELECT dateTime, rain FROM archive WHERE dateTime > 1492725600 and dateTime <= 1492812000 AND rain > 0;" | sqlite3 weewx.sdb
(NB: 1492725600 is midnight 21-04-2017, and 1492812000 is midnight 22-04-2017 in the time zone UTC+02:00.)