In sandbox mode, you can take the following steps to test your dunning management configurations.
1. Subscribe an account using the "Success" test card number: 4111-1111-1111-1111
2. Update the account's billing information to use the "Fail, but save" test card number: 4000-0000-0000-0341 (note: it will appear this action returns an error, but this is expected. The card number will still save on the account).
3. Speed up the renewal process by going to the subscription table of the account then hovering over "options" and "view subscription." There you can then "Change" on the subscription details page.
4. Set the subscription's next renewal date 1 hour in the future.
5. The subscription will renew within the hour, kicking off your dunning process.