Once in the editor, click on 'Choose' next to the Invoice number:
A pop-up will open where you can change the prefix and set your invoicing numbering sequence. For more details about how to set/edit invoice numbering sequences, please refer to our Invoice numbering article.
Note that you have to approve the invoice in order to save the new sequence. If you save your invoice as a draft, you will have to enter again the new sequence before approving it.