Sub zero()
Dim ws As Worksheet
For Each ws In ThisWorkbook.Worksheets
If InStr(1, UCase(ws.Name), "MR") > 0 And _
(InStr(1, UCase(ws.Name), "MR") <> InStr(1, UCase(ws.Name), "MRS")) Then
If UCase(ws.Name) = ws.Name Then
ws.Name = WorksheetFunction.Substitute(ws.Name, "MR", "MRS")
Else
ws.Name = WorksheetFunction.Substitute(ws.Name, "mr", "mrs")
End If
End If
Next
End Sub