You cannot change the bonus ticket you've already issued. If you want to change your schedule, you need to get a new ticket after refund.
Bonus ticket refund procedure is as follows:
After refund process in the Jin Air homepage ▶ My Page ▶ Ticket Refund, please go to the [Membership > Nabi Points] and move to [Ticket Menu]. Then, please click the [Bonus Retrun].
For your information, if you do not use the bonus coupon, you can only get a full refund and partial return or reservation change are not allowed after boarding some sections.