Unless exceptional circumstances or agreement of the parties the new order can only go back to the date that the Complaint for Modification; which is the paperwork that you’re saying that there is a change in circumstances was served. Served means the date that a process server, constable or a sheriff serves the other parent a copy of the Summons and Complaint.
It does not go back to the date that the change occurred, such as a loss of a job or health insurance changed. Even if you can prove that it happened, it only goes back until to the date that person was served. It is important to file and get the other party served because it will only go back to that date.
You can also file something that is called “Joint Petition or Motion to Change a Judgment or Temporary Order” that goes back to the date that you filed that order.