I mistakenly issued a Credit Note in the wrong tax year.

Unfortunately, this can't be undone or deleted. The best thing to do in this instance is to create an SP Ledger entry for the same date that reverses the bad credit note. When this has been done, recreate a new credit note with the correct date entered.

The short video here, Record an Entry Using the SP Ledger, details how to use the SP Ledger.