There is no way to retroactively apply a coupon to an order that's been placed.
If the coupon you hoped to apply is limited to the initial payment only, some payment vendors allow you to issue partial refunds from within their administrative dashboards. You can check the documentation for your payment vendor to see whether this is possible, and how it might be accomplished. Note that our platform will not be informed that the partial refund occurred, so doing this would introduce some inaccuracy to your MemberMouse Reporting and Analytics data.
However, if the desired coupon would apply to all subscription payments, your only option is to cancel the member's subscription, issue a refund, and have the member purchase their subscription again, making sure to utilize the coupon.