If a person forgets to say “Mashiv HaRuach U’Morid Hageshem”:
1) If he remembers before the end of the Bracha, then he should state it in the place he remembers about it and continue from that place (BUT he should not split up a phrase. For example, if he remembers after he says the words U’mekayaim Emunaso, he should first say L’Shainey Afar before stating Mashiv Ha’Ruach)
2) If he remembers after saying Veneeman Attah, but before stating Hashem’s name in the Bracha, he should state Mashiv HaRuach, and then continue from Veneeman Attah.