You can find Change Orders in the Contract and Change Orders section.
First, add a change order to your project. You can learn how to create a Change Order by clicking here
Making the change order active will allow you to click the gear icon next to the change order, this will open a drop down menu. Clicking Create phase in plan automatically will bring the change order in the Plan & Track as a job phase.
Once the change order phase is created in the Plan & Track you can treat it as you would any other job phase. As well as track any resources needed.