You can’t.
The halacha is primarily that a woman cover her head, not her hair.
The prohibition is for a woman to go out ???? ???? – “her head uncovered” (this is the translation of the Rishonim). The Gemara in Kesubos which introduces this halacha does not mention the word hair.
See my posts here: http://www.theyeshivaworld.com/coffeeroom/topic/hair-covering-and-married-women