If you have a non-changeable air ticket, you can not make any changes. You need to cancel it and reserve new one.
You can cancel by calling our customer service or submitting a cancellation form below.
Cancellation form
https://www.airtrip.co.jp/support/japanflight/cancel_form_expedia_en/cancelform.php
If you have a changeable air ticket, you can change departure date and flight if it's same airline, route and passengers.
You can change by calling our customer service or submitting inquiry form below.
Inquiry form
https://www.airtrip.co.jp/support/japanflight/contact_form_expedia_en/
Customer servise：03-6635-3898
【Business hours】7 a.m. - 9 p.m.