Sub ChangeSubjectSelection()

Dim selectionIndex As Long
Dim itm As Object

For selectionIndex = 1 To ActiveExplorer.Selection.Count

    Set itm = ActiveExplorer.Selection(selectionIndex)

    If TypeOf itm Is MailItem Then
        itm.Subject = "add value here-" & itm.Subject
        itm.Save
    End If
Next

End Sub

Dim Item As Outlook.MailItem
Dim oInspector As Inspector
Set oInspector = Application.ActiveInspector
Set Item = oInspector.CurrentItem

If InStr(Item.Subject, "New Value -") > 0 Then

Else

    Item.Subject = "New Value -" & Item.Subject

End If