It's very easy to apply a discount to an existing subscription whether it's for a customer support issue or to fix a pricing issue. To get started, press the "Apply discount" button under the list of products in a subscription:
You'll see the options to create a one time, specific number of instance, or indefinite discount:
Once you choose an option and set the amount, you'll see the discount applied to the subscription. If the discount is only one time or for a specific number of instances, it will be automatically removed after the period is over.
If you wish to apply a discount to a specific product, you can do that by removing the product and re-adding it with a discount. See the article on adding products for more details on how to do this.