Subscription PromoCode is not editable after BA 6.0.6.
Ability to manage "Promoted by" field value for subscription was removed as a part of the fix of product bug #PBA-66405.
Feature Request #PBA-68267 (Possibility to edit the "Promoted by" field for subscription) submitted to add back this functionality.
An instaruction to remove a promotion from the subscription is described in guide:
"To return regular fees in the current subscription, you need to renew the subscription without a promotion, that is to leave the Promotion Code field empty, or just switch the period."