If you have deleted or edited a coupon that was used for previous registrations, this will not affect your previous sign-ups. The current coupon settings will be applied to any new registrations.
For Stripe
If you are looking to edit a coupon that previous students have already used due to incorrect coupon settings, then any updates will need to be made in Stripe. For example, if you meant to create a coupon that only applied to the first payment in a subscription, but it is currently being applied to all payments, you can manually remove the coupon in your Stripe account.
For Thinkific Payments
If you have migrated to Thinkific Payments as your payment processor and want to apply coupon changes retroactively to your existing subscription, follow the steps here: Add a Coupon to a Subscription