My wife doesn't dress decently. I have tried to make understand but she is not paying heed to it. I don't know what to do. please guide me in this regards.
In order to address this issue please address the root cause. Dressing decently is an offshoot of lady's modesty and chastity (hayaa). Unfortunately, today hayaa has been lulled to sleep. But it is present in a lady in dormant form even when she dresses indecently. The correct approach is to awaken this all important faculty in her. The best thing perhaps to do is to not directly force her or argue with her to wear a proper dress. In this regard, we should try to connect such a lady to the Qur'an which perhaps can provide the biggest boost to hayaa and is powerful enough to awaken it. So try to convince her to read a good translation of the Qur'an.
At the same time, you should stop discussing the dress issue with her, and do pray very deeply to God to make her change.
So the Qur'an and dua may hold the key to this issue. Also remember that she is an adult. You should not force her as she is accountable to God for herself.