If you realise you have made an error but the booking has already been invoiced, you can credit the invoice from the context menu on the Invoice form by choosing the Credit option.
This will include the events from that invoice on the next invoice, along with any other events that match the criteria.
If you want to raise a "pure" credit note, rather than including other bookings, you can do that by entering
in the Additional Criteria area at the bottom of the Generate Invoices modal dialog:
If your browser has a spell checker or you have a browser extension that does, you will get a red wavey line like in the screenshot - you can ignore that.