On the book a flight page, to the right you will see "Quick Air Links." Under that you have 3 options.
Check In
Change Flight
Check Flight Status
Click on "Change Flight"
It will ask you to enter your reservation information. Once you do that, your current flights will pop up.
Check the box next to the flights you want to change then press continue. After you will continue just as if you were booking a new reservation. At the end, you will see the credit you were given.
Keep in mind, you have to use (fly) on or before the day you first booked the flight. Not from the time you modified it. It will tell you in the new confirmation email when the credit expires.
Also, don't lose the confirmation #. The credit will be tied to that number.