When you create an order it is possible to change the reference number. If you do not edit the reference, it will default to the next order number in sequence.
To change the Reference Number select the pencil icon side Reference Number and free form in any numbers. Note: you need to select a unique number, it is not possible to duplicate reference numbers.

After you change the number in the reference number box, it will update the reference number at the top of page and on the main order index page.
Note: you can only change the reference number when the order is in draft status. Once Accept has been selected it will lock in the reference number.

VIDEO: How to Change an Order's P.O. Number, Reference Number & Date