Old invoice lines cannot be deleted because the invoices have to have consecutive numbering. Consecutive numbering is determined by the accounting act so the only option is to make a credit note.
A credit note can be made by opening the invoice from the Incomes section and clicking the ”Actions button", or the 3 dots shown on the right. After that you should see "Reimburse” on the list. If the original invoice has been sent as an e-invoice, email-invoice or as a standard letter, the credit note will be sent using the same method.