Hijab is a duty Allah, the Almighty prescribed for the Muslim woman, and she has to comply with that order and show her sincere faith in Allah, for He says: “And it does not become a believing man or a believing woman, when Allah and His Messenger have decided an affair (for them), that they should (after that) claim any say in their affair.” (Al-Ahzab: 36)
When a Muslim neglects to dress according to the Islamic code it is usually because of lack of knowledge and weakness of Iman. Negative influences from her environment can further dissuade her from obeying Allah, the Almighty. Things like media, literature, friends and socialization.
If you want to change someone’s life, you should start with the afore mentioned influences. Usually, when you see any weakness or neglect in a person’s readiness to obey Allah, you should know that there is some bacteria or virus that requires remedial work.
In Islam, teaching and discipline are the first steps in guiding someone.

In this regard Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states: The husband in Islam, according to the Qur’an, is considered the leader of the family, and therefore it is his duty to make sure that his family, including his wife and children and those under his care and responsibility, remain steadfast as much as possible on the straight path. Therefore, a husband is right in asking his wife to wear Hijab. By asking her to wear hijab he is not dictating his personal whim upon her, rather he is reminding her to comply with the Command of Allah (please refer to verse number 31 concerning Hijab in Surat An-noor.)

Having said this, I must say that a husband should have approached his wife in a gentler way especially if she has not been observing Hijab before marriage. Allah Almighty commanded Musa and Haroon, peace be upon both of them, to preach Pharaoh using the most gracious and gentle approach. Therefore, a husband should not be harsh with his wife and he is not justified in being rash on breaking the marriage suddenly on this ground. He should give her ample time in order for her to understand the wisdom of Hijab.

If a husband has already advised her wife then he has discharged his obligation and he is not going to be accountable if she has failed in carrying out her duty to observe Hijab. After all, in Islam each person is ultimately responsible for his or her own salvation.”

Also, the erudite Muslim scholar of Bahrain, Sheikh Nizam Ya`qubi, states:
“First of all, it should be clear that Hijab is an obligatory duty and every Muslimah is required to wear it. There is no disagreement am
ong all Muslim scholars to this fact and Allah has made it clear in surat an-Nur, “And tell the believing women to lower their gaze and be modest, and to display of their adornment only that which is apparent, and to draw their veils over their bosoms, and not to reveal their adornment save to their own husbands or fathers or husbands’ fathers, or their sons or their husbands’ sons, or their brothers or their brothers’ sons or sisters’ sons, or their women, or their slaves, or male attendants who lack vigour, or children who know naught of women’s nakedness. And let them not stamp their feet so as to reveal what they hide of their adornment. And turn unto Allah together, O believers, in order that ye may succeed” (An-Nur: 31)
Therefore, you must continue to convince your wife with this fact using all available means. If she accepts the judgment of one of the respected scholars in your area, then take her to them. If she is well-read in Islamic matters, then you can present to her relevant Islamic books.
Finally, make du`a’ (supplication) for her that Allah guides her to the straight path.”