You create a credit in exactly the same way as creating an invoice. 

Enter details as per usual and then enter a negative quantity, either by clicking the down arrow or typing a minus sign (-) before the number. 

This will deduct the credit from your total.

Click + to add another line or x to delete. Click Save when you are done.

How do I create an invoice?
​How do I mark an invoice as paid?
​How do I amend a payment on an invoice?