You cannot change the origin airport
You can only change the destination airport to one in the same country (there is some tax issue involved based on what I found out years ago which the website cannot work out/audit)

Call and ask, on the phone it should be possible to manage a change of airport

If the circumstances warrant it there is a special bereavement fare you can ask for if you find the cost to change excessive

Had you purchased a return ticket or a muitcity return then this would be a lot easier...