It will really depend on what we are disagreeing on but for the most part my approach will be to try to understand what is causing the disagreement. As much as you are eager to make your case, take a step back and try to understand the perspective your manager has on the topic. Make sure you hear your manager out and understand the rational. You can then take your turn to explain your view and the rational behind it. With the right intentions on both sides you should be able come to a reasonable position. At any point of the conversation, you should make sure you are not undermining the accountability that rests with your manager. It should never turn into I know better you should do what I say kind of conversation. It is always best to remember your role is to help your manager and the team do the best work possible for the product, the team and the people in in the team. It is never about being proven with the best idea. If you disagree with the managers decision, you should register your disagreement but as long as you are in the role you need to execute on the direction provided as provided it does not violate your morale campus and company policy.