How do I refund if the credit card used is cancelled or changed?

From Stripe:

In the majority of cases, refunds made to closed or canceled cards should still succeed. Typically, if the customer has a new account with the same bank, the bank will just push the credit over to the new account. This may take a little extra time (on the order of days or weeks), but if the customer still has an account with that bank, it should eventually get there. The support representatives at your customer's bank may or may not have insight into the movement of these funds during that time.

And even if the customer has no current account with the bank, most banks are still able to figure it out. We’ve even heard of banks cutting paper checks to their former customers in cases like these.

If the bank is not able to push the refund through on their end, they’ll typically return the funds to Stripe within 30 days of the refund date. We’ll notify you by email if and when they do, and return the funds to your Stripe balance.

You can read more about refunding canceled cards here:
https://stripe.com/docs/refunds#destination

You can learn more about frequently asked questions (FAQs) about refunds here : https://support.stripe.com/questions/refunds-faq

If the refund ends up not going through, Stripe will send you an email and the funds will be dispersed back to you for you to divvy out to the guest.