Your merchant account acts as a buffer in the event that transactions are contested via chargebacks. When a customer subscribes to your service, the credit card number is sent from Recurly through your payment gateway and finally to your merchant bank account.
More information on transactional flow can be found here: https://docs.recurly.com/docs/gateway-merchant-account-overview#section-transaction-flow
For security purposes, since your merchant account deals directly with revenue, its completely separate from your Recurly Account. To update or change this account information / add a new account you’ll want to reach out to your payment gateway directly (i.e. Stripe, Authorize, SagePay etc.) for details on how to process your update, or by logging into your payment gateway portal.
With Configuration permissions you can view you payment gateway(s) here: https://app.recurly.com/go/configuration/payment_gateways and reach out to your payment gateway(s) to access and update your Merchant Account information.