How do I get a refund for a cancelled subscription

See this Apple article: Billing and Subscriptions

Subscriptions and Billing - Official Apple Support

View your puchase history,

Cancel your subscription,

Try to Request a refund.

Note that requesting a refund does not guarantee you will get one. Also, depending on the original payment method, it may take up to 30 days to get your money back even if Apple does decide to give you a refund.