how do change crystal report field with code

If that date column is coming from a database, you can do something like this

recordSelection.AppendFormat(
 "{{table_name.sale_date}} >= {0} and {{table_name.sale_date}} <= {1}",
       startDate.ToCrystalString(),
       endDate.ToCrystalString());

.ToCrystalString() is a custom method I have used to change the sql datetime to crystal date time for my use. You can make a method of your own to convert to persian date.

Let me know if you need further assistance.