Rebooking Airline tickets

Call the airline. If they were made less than 24 hours ago, they can be cancelled--even non-cancellable tickets. Sometimes this can be done online.

If more than 24 hours, it depends on the type of ticket and the airline. Call and ask.

If they were purchased through a third party, you have to deal with the third party.

Good advice above. If, for some reason, the first person says no, hang up and call again.

You should be able to get some idea of the refund and rebooking conditions
of the ticket in the email you received from AF when you booked.

The sooner you get back to the airline, the more chance they will believe you
booked the wrong dates by mistake and not that your plans changed.

If you are having trouble getting through to the airline on the phone, try Googling
the # for AF in France or some other European country and call that number during
late night European hours. On something like Skype or Whatsapp the cost will
be minimal.

Your ticket, unless it's some variant on Basic Economy, should have residual value
of some sort under most circumstances. The question is whether you can rebook,
or whether this is treated as a full-out cancellation and then a new booking. If
the fare has gone down since you purchased, it may work out OK.