A family of Microsoft word processing software products for creating web, email, and print documents.
Word supports multiple underline formats, and therefore there is no simple toggle available.
Using an IF statement, you can construct a macro that toggles between single underline and no underline at all:
If Selection.Font.Underline = wdUnderlineNone Then
Selection.Font.Underline = wdUnderlineSingle
Else
Selection.Font.Underline = wdUnderlineNone
End If