Can I appeal a prior year’s assessment?

Generally no—you can't file a regular appeal for a prior year after the deadline passes. But a Certificate of Error may correct a verifiable factual mistake (wrong square footage, non-existent/demolished structure, misclassification, wrong unit count), applying to the current year and up to three prior years, possibly with a refund. A COE can't be used merely because you think the value was too high or you missed the deadline.