Once the month has been closed you should no longer be making changes to the billing.
However there are cases eg when you need to supply a different ICD10 code on a claim, when you will need to do this
In System variables there is a switch . check box that says 'Allow changes to a closed month' - switch this on while making the changes and then switch it off again.
See Help files for detailed instructions and warning regarding making financial changes after the month has been closed.