If you generated a draft by mistake, there’s no need to correct anything. Just undo it: the appointment goes back to pending, as if it had never been invoiced, and you can invoice it again whenever you like.
In short
A draft (in blue) is undone with the back arrow on its row. The appointment goes back to Pending. An issued invoice isn’t undone: it’s corrected.
Find the draft
In Invoices → From Appointments, drafts show the blue Draft status and their row has the view, download, edit, tick and undo icons.
Press the back arrow
Press the back arrow on the draft’s row. Its tooltip reads Unmark as invoiced.
The appointment is pending again
The draft leaves the Draft status and the appointment goes back to Pending. You can select it again and generate its draft when you like.
Undo or correct?
Undo is for drafts you haven’t issued: no document is left behind. Correct is for issued invoices: a corrective invoice is created and stays on record. See the guide Correct an issued invoice.