Praise be to Allah.
Obesity may be due to a kind of sickness or hormonal imbalance in the body. Treating that should be referred to specialists.
And it may be due to overeating and not adhering to the etiquette prescribed by Islam. The way to treat that is by always mentioning the name of Allah before eating, and praising Him after eating, and reducing one’s intake of food. At-Tirmidhi (2830) and Ibn Maajah (3349) narrated that Miqdaam ibn Ma‘di Karib said: I heard the Messenger of Allah (blessings and peace of Allah be upon him) say: “A human being fills no worse vessel than his stomach. It is sufficient for a human being to eat a few mouthfuls to keep his spine straight. But if he must (fill it), then one-third for food, one-third for drink and one-third for air.” This hadith was classed as saheeh by al-Albaani in Saheeh at-Tirmidhi.
Allah, may He be exalted, says (interpretation of the meaning):
“and eat and drink, but be not excessive. Indeed, He likes not those who commit excess”
There is no specific remedy in the Qur’an and Sunnah for the problem of obesity, even though the Qur’an is real healing, as Allah, may He be glorified and exalted, says (interpretation of the meaning):
“And We send down of the Qur'an that which is healing and mercy for the believers, but it does not increase the wrongdoers except in loss”
“O mankind, there has to come to you instruction from your Lord and healing for what is in the breasts and guidance and mercy for the believers”
Ibn al-Qayyim (may Allah have mercy on him) said: The Qur’an is complete healing from all diseases, both psychological and physical, and offers protection this world and the hereafter, but not everyone is qualified or will be successful in seeking healing from it. But if the sick person seeks treatment through the Qur’an and is able to use Qur’anic remedies with sincerity, faith, complete acceptance and certainty, fulfilling all the necessary conditions, no diseases will ever stand up to it.
Zaad al-Ma‘aad, 4/322.
It is prescribed for one who is sick to recite al-Mu‘awwidhaat [soorahs seeking refuge with Allah] and to blow over himself. That will have a beneficial impact, by Allah’s leave.
Al-Bukhaari (4439) narrated from ‘Aa’ishah (may Allah be pleased with her) that the Messenger of Allah (blessings and peace of Allah be upon him) used, when he fell sick, to recite al-Mu‘awwidhaat [soorahs seeking refuge with Allah] into his hands, then blow into his hands, and would wipe himself with his hands. When he fell sick with his final illness, I recited al-Mu‘awwidhaat [soorahs seeking refuge with Allah] over him and blow over him, and wipe his hand over him.
Muslim (2192) narrated it as follows: If one of his family fell sick, the Messenger of Allah (blessings and peace of Allah be upon him) would blow over him and recite al-Mu’awwidhaat. When he fell sick with his final illness, I started to blow over him and wipe him with his own hand, because it was more blessed than my hand.
Al-Bukhaari (5018) narrated from ‘Aa’ishah that when he went to his bed every night, the Prophet (blessings and peace of Allah be upon him) would put his hands together and blow into them, then recite into them Qul Huwa Allaahu ahad, Qul a‘oodhu bi Rabb il-Falaq and Qul a‘oodhu bi Rabb il-Naas [i.e., the last three soorahs of the Qur’an), then he would wipe his hands over as much of his body as he could, starting with his head and face, and the front part of his body. He would do that three times.
It is also prescribed for the Muslim to pray to Allah, may He be exalted, for whatever he wants of the good of this world and the hereafter, and to avert from him what he dislikes. So ask Allah, may He be exalted, for healing and well-being, and for goodness and beauty.
And Allah knows best.