As long as the parking transaction is not on the invoice, you can change the assignment.
Go to My Pages. Tap on the three menu lines in the top right corner and select 'Parking Tickets/ Receipts'.

Search the list for the parking transaction for which you want to change the assignment and scroll all the way to the right in the last column. Tap on the assignment and then on 'Change Assignment'. Check if your selection has been applied and that the assignment is displayed correctly.