You can add a coupon code to an existing subscription in Stripe. You will need your coupon code setup before this step. You can see instructions for this here: How to create Stripe coupons
- Go to https://dashboard.stripe.com/dashboard and search for your customer name. Select your customer
- Scroll down until you see the subscription. Click the 3 dots next to the subscription and select Edit subscription.
- On the 'Edit subscription' page click the 'Add coupon' button next to Discount. Add the coupon code
- Scroll down to the bottom of the page and click Update subscription.