You'll first need to head to the desired customer, and their active membership.
Tip. Find the customer from your customer list and click on the membership below their name to take you direct to the right page.
From here you will have the option to add a discount code.
Simply select the desired code from the drop down list, and click 'Add Discount' once you've selected the right one.
If you need to add a code first, click here to see how to do so.
Note. If the discount code you've applied is valid for a certain number of billing cycles, it will apply the code to as many cycles from the moment you add it.