There are two types of credits that are normally issued: full credit and partial credit.

For full credit, follow below:

1. Go to the selling invoice that you need to issue credit for.

2. Go to the "orders" tab.

3. On the right upper side of the screen, you will have a "credit" button:

4. Once you click the credit button, the system will generate a credit invoice and all orders will showing in negative amount:

5. The credit invoice will be created in status "finance approved" and will have a number auto generated.  It will also show in the reference field the number of the selling invoice that was credited.

To issue partial credit: 

1. Go to the economics tab > selling

2. Mark the orders you wish to credit and click on credit invoice

** It is also possible to issue credit through the SO.