The contested hearing is where you present evidence to the court (see below), and the court decides whether to change the final order.
First, make sure that the hearing that is scheduled is, in fact, a contested hearing, or court trial, or final hearing, where evidence will be presented. If it is not clear from looking at the notice, call the court and ask the clerk.
At the hearing, the court will give each side a turn. If you filed the motion, you will go first. You will present all of your evidence about why the order should be changed. The party on the other side will be allowed to question you and any witnesses that you bring to the hearing. Then the other side will get a chance to show evidence about why the order should not be changed, and you can question his/her witnesses.
You have to prove, using evidence, everything that you have said in your paperwork. The court cannot rely on what you said in your paperwork. You will need to prove that there has been a major change that you did not know would happen at the time of the final order. You will also need to prove that changing the order is in the child or children's best interest.