How do i change the data format of a column of a table?
I need to show an error message if data in date of birth textbox is entered incorrectly, need to specify the format as well. Will it be done through database or visual studio? How?

You can run a function on your value that runs a regular expression on the date entered.

This returns a boolean value. Throw an error if the date does not pass the test. Write your error to a label, "Please enter date as mm/dd/yyyy."

    Function CheckDate(ByVal Value As String) As Boolean
        Dim ex As Regex = New Regex("([1-9]|1[012])[- /.]([1-9]|[12][0-9]|3[01])[- /.](19|20)\d\d")
        Return ex.Match(Value).Success
    End Function