How much weight will you lose if you drink water instead of soda and walk on the treadmill for 30 minutes 5 days a week?

Cannot be answered if we do not know how much soda you replace.

If it's 3 12-oz Cokes a day, that's say 500 calories; about 100 calories if you are 150 pounds when walking the treadmill for 30 min, so about 600 day; you need to save about 3000 cal to lose a pound. This means in 5 days, you might lose a pound.

Keep at it.