All you have to do to modify your Southwest Airlines trip online is -
On the booking widget at Southwest Airlines, select the Change/Cancel tab.
Click on the "Change flight" option after entering the passenger's last name and the six-digit ticket confirmation number.
To access your booking, click the "Search" option.
Select the flight schedule that needs to be modified by clicking. You can alter the departure city, the departure date, and the departure time.
All flight changes are subject to the Southwest Airlines change flight fee, per the airline's flight change policy.
At the checkout page, use a debit or credit card to pay the necessary Southwest fee to change flights.
Once the modifications have been made, the traveller will receive a confirmation of the changed booking along with the updated flight itinerary at the email address they have on file.
Also visit : Jetblue Airlines change flight fee