How can i gain muscle fast with dumbbells?

That depends on a few "ifs": how heavy the weights; how often you exercise with them; how many repititions. Understand that building muscle is a form of healing: in order to build muscle one must tear down muscle which heals back with added muscle mass. Bells are generally used for toning, not body building, because, unlike bar bells, weight is harder to increase. So rather than increase resistance weight you are left only to increase frequency and duration. And there is a practival limit to time available for that.

Workout your muscles and eat more. With dumbbells, you can do various exercises like curls, bench press, flies, shoulder press, squats and so on. Workout your entire body. If you can use, a weight that is heavy enough for you to do 8-12 reps max for 4-5 sets. Remember, your diet is crucial. Consume more protein and calories.

Workout out often, consume lots of protein, exercise different muscle groups each day so you don't overwork your muscle

30 reps a night, 30 in the morning, double your protein intake and halve your carb intake