At this age, your child could be getting a little stubborn and every parent of a strong willed child knows that it can be a challenge raising one. You may find it very difficult to break her nature and mellow her down especially when she's not easily swayed from her view point and gets into power struggles till you give in.
However, strong willed children generally grow up to be teens and adults who are self-driven, motivated and unaffected by peer pressure. So, don't worry there is a way of turning this around!
What you can do
You can make this really interesting by letting your kid participate in the decision-making process!
Make every decision look like it has been taken by her. If she tends to resist when given a command, try seeking her advice first. Make her reason out the options aloud and show that you understand her dilemma. Talking to your child in a respectful manner can do the magic. Stay calm and maintain a strong connection with her. Try not giving your child a sense that she has the ability to frustrate you with her arguments because that can lead to her becoming even more headstrong about it.
Life's full of making choices and this way she's learning some decision making skills early!