It depends entirely on the fare you pay. Usually the absolute cheapest allow no changes whatsoever, whereas somewhat more expensive ones will allow changes once you have started your trip. Also, you should be aware that changes are usually only available for the specific fare class you paid, if there is no availability in that fare class you will have to pay (i) a change fee and (ii) the fare difference between one and the other (not an upgrade).

It would help if you told us where you are travelling from and to.