Select the customer in Stora and then use the 'view on stripe' button. On the customer's Stripe page select '...' and then Update subscription as shown below.

Select the three dots to the right of the current discount, and click on edit. This will allow you to change or add the discount to the desired code.

OR, If the subscription currently doesn't have any discount you can select "Add coupon" to add the discount code.

Select the applicable code from the drop down. These are the discount codes that have been created in Stora under the Discounts tab.

Ensure that Reset Billing Cycle and Prorate Changes are unchecked.

When you have selected the code you require, select Update Subscription. The summary in the right of the page will also show you the new invoice amount. Please review this before submitting your change.