Dear Omprakash
you can use following macro
select the cells you want to make changes in comment & run this macro.
Please dont forget to experiment this with on small part of file or
backup copy.
i have written the code just now & tested it but not sure of any bug


Sub ReplaceinComment()
Dim temp1 As String, temp2 As String, temp3 As String, temp4 As String
ians1 = Application.InputBox("Find what", "Find & Replace for
comments", "")
ians2 = Application.InputBox("Replace with", "Find & replace for
comments", "")
temp3 = ians1
temp4 = ians2

For Each cell In Selection
temp1 = cell.Comment.Text
On Error Resume Next
temp2 = Replace(temp1, temp3, temp4)
cell.Comment.Text Text:=temp2
Next
End Sub

Regards
NC