Here is a link describing the entire process of how to apply a discount code in ReCharge to an EXISTING CUSTOMER (they've already placed their subscription order)
Note that if you must create the discount code in ReCharge for this to work. ReCharge cannot pull discount codes from Shopify. For example, if you want to give someone 10% off their existing subscription, you couldn't use a Shopify 10% discount code. You'd need to create the discount code in ReCharge, then go through the steps outlined in the article above.
You can create a discount code by clicking on the "Wrench" icon in the top right corner, and clicking "Discounts."