My 5yo daughter has become very naughty and she has started back answering already. How should I discipline her?

Generally this age kids will try to do a lot of objectionable things to get attention towards them. Each time she back answers, your reaction to it is what she looks for. Each time u react strongly, she feels like she has won a battle as this is the only thing she has control over. Don't react to her. But don't encourage it too. Try to repeat whatever she said in the same tone. Watch her reaction. Read or create stories where the same scenario is shown where moral of the story turns out positive.