A father can overturn the nedarim [“hafaras nedarim“] of his daughter until she matures. That is, until she is 12 years and 6 months old; and that is if she doesn’t marry. A husband can overturn the nedarim of his wife.
How do we overturn? He says three times “overturned” or “canceled”, or any other language which indicates that he is uprooting the neder from it’s roots — whether if he says so in front of her or not in front of her.
However, with a language of “release” [“hatars nedarim“] the father or husband have no power. Also, they can not overturn the neder except on the day they hear of it. That is, if they hear in the begining of the night, they can overturn [the neder any time during] the entire night and the entire day that is after it. But if they hear during the day, right before the stars come out, they can only overturn [the neder] until the stars come out, and after that they can not overturn it.
On Shabbos he should say to her “it is overturned for you” was he would on a weekday. Instead he should anull it in his heart and say to her “go and eat” [if the vow was not to eat something], and the like.
If the father or husband first said that they desire this neder, even if he does not say so explicitly but says some language which indicates that he desires it, even if he only thinks in his hear that he desires her neder, he can no longer overturn it.
I discussed the concept behind this in the past. Just the final conclusion without the justification, to tease you into chasing that link:
By explicitly using the term ishahh rather than baalah when discussing the anulment of vows, we see that the husband has the power of hafaras nedarim not in his role of provider and therefore holding control (as anyone who holds the purse-strings will), but because it’s his role in the partnership to be the one who sets new directions, just as it is hers to insure that they are developed in a holy way.