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You can change the validation rule for a range by using either of the two ways described below:
Set properties of the IValidation ( IValidation.Type , IValidation.Formula1 , IValidation.Formula2 , and many more).
Use IValidation.Delete of the IValidation interface to first delete validation rule and then use the IValidation.Add to add the new rule.
Refer to the following example code to know how you can modify an existing validation rule applied to a cell or a range of cells in a worksheet.
//Add validation
worksheet.Range["A1:A2"].Validation.Add(ValidationType.Date, ValidationAlertStyle.Stop, ValidationOperator.Between, new TimeSpan(13, 30, 0), new TimeSpan(18, 30, 0));
//Modify validation.
worksheet.Range["A1:A2"].Validation.Type = ValidationType.Time;
worksheet.Range["A1:A2"].Validation.AlertStyle = ValidationAlertStyle.Stop;
worksheet.Range["A1:A2"].Validation.Operator = ValidationOperator.Between;
worksheet.Range["A1:A2"].Validation.Formula1 = new TimeSpan(13, 30, 0).TotalDays.ToString();
worksheet.Range["A1:A2"].Validation.Formula2 = new TimeSpan(18, 30, 0).TotalDays.ToString();