Replacing sugary drinks, juices and alcohol with water removes a significant source of calories for many people, which is the main way water helps with weight. Drinking a glass of water before meals can also slightly reduce how much some people eat, and staying hydrated supports energy and exercise.
Water does not burn fat or speed up metabolism in any large or lasting way, and drinking large amounts will not compensate for an overall calorie surplus. Thirst is sometimes mistaken for hunger, so having water available can help you respond to genuine hunger rather than habit.
Drinking excessive amounts of water in a short period can rarely be dangerous by diluting blood sodium, so the aim is sensible hydration, not extremes. Combine adequate water with a balanced diet and activity, and seek advice from a doctor or dietitian if you have a condition that affects fluid intake.