You can make changes on the phone by quoting your booking reference number. Any charges and fare differences will be charged to your credit card. They will let you know the amount before agreeing to changes and charging.

OR

Any QA office will make the changes for you. Try airport branch. The changes might be free, or charged, depending on your ticket fare. If it's a special offer ticket, changes might be restricted.

Also remember to carry your credit card when flying. You need it for check-in.