Log in to your account and open the reservation you want to change. Select the “change” option and select the flight or fare you want to change to. From there, just follow the directions and confirm you want to make the change. If you did not purchase with points, the fare difference is held in your account to be applied to a future trip, with travel completed within one year from original purchase date. If a points purchase, the points will be redeposited into your account, with the option to refund taxes/fees (which I do) or held for future purchase.