Wanting to change my child’s diet

I’m needing help and suggestions on starting to remove high sugars, red dye, and whatever else in my four year olds diet because I’ve been told that may help with her behavior. If you have done this where do I even start? What are good affordable snacks and drinks? I know this is going to be difficult because she is very spoiled and is so use to eating whatever and getting handed candy all the time but I’m willing to give this a shot if there’s a good chance it can help with some of her behavior.