how do you change the data type of one cell in a colomn on crystal report

That would work.

You could also use a formula like this.  If you wanted say the 10th record to use a text field instead of a number field

If RecordNumber = 10 then
    {YourTextField}
Else
   CStr({YourNumberField})

mlmcc

Place the field on the report canvas,
Right-click, Format Field, Common Tab
Enter an expression for the 'Display String' property, such as:
IF NumericText({file.REFERENCE}) Then
     ToNumber({file.REFERENCE})
Else
     0;

Thank you all for your quick responses. Let me add a little details: what I am trying to do is
I imported an excel file into crystal report. everything went well except that one row of each column has different data type than the rest of the documents. It has to be date data type rather than number data type in order to show correct info on the report. It should look like this:

Jan-12         Jan-13        Jan-14      Jan-15      Jan-16
123                456             456          786           555

right now the dates row shows as a number:  42156 which is the crystal conversion of the month and year into a number.

I appreciate all your help, but feels easier to try mlmcc's and idomillet's. will update the result

A date in Excel is actually a number disguised as date.  
It is the number of days since January 1, 1900

You can reverse that logic in Crystal using a formula such as:
DateAdd('d', {your_excel_date_column}, #1/1/1900#)  

For example, DateAdd('d', 42017, #1/1/1900#) returns January 15, 2015

You can test to see if a value is a Date using the IsDate() formula.

i tried to use both mlmcc's and idomillet's methods, but didn't work. Would you please be more specific on how to set first row of each column as date data type and rest of the the columns as numeric data type?

thanks!

mlmcc already gave you the solution for that:

If RecordNumber = 1 then
    {YourTextField}
Else
   CStr({YourNumberField})

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If you need a date as first row then numbers after that
Eric's formula needs a minor change.  You need to convert both to strings

If RecordNumber = 1 then
    CStr({YourField},"dd-MMM-yyyy")
Else
   CStr({YourField})

Select allOpen in new window

Add that formula to the report then put it where you want the data displayed.

mlmcc

Do you need to use that date as a date value (eg. to compare it with another date), or do you just want to show the date on the report?  Similarly, do you need to use the number as a number (eg. to calculate a total), or do you just want to show it on the report?

 If you just want to show that value (date or number) on the report, you can use a formula like the last one that mlmcc posted, but you need to convert the number to a date first, and then convert that date to a string.  Combining mlmcc's and Ido's suggestions:

If RecordNumber = 1 then
  CStr (DateAdd ('d', {your field}, #1/1/1900#), "MMM-dd")
Else
  CStr ({your field})

 You can change the CStr arguments to change the date or number format.


 Assuming that Ido was correct about how to convert the Excel number to a date, a slight variation on that would be:

If RecordNumber = 1 then
  CStr (Date({your field} + 2), "MMM-dd")
Else
  CStr ({your field})

 The CR Date function will convert a number to a date, but for some reason it uses the number of days since 12/30/1899, instead of 01/01/1900, so you need to add 2 days.

 As Ido mentioned, if there's any doubt about the numbers giving you valid dates, you can use the IsDate function to test them first, to avoid getting an error.

 James