Every fare carries its own change rules. You can read them on the booking before you pay, and again on the confirmed booking.
Typically a change costs the airline change fee plus any difference in fare between the old and new flight. Deeply discounted fares are sometimes not changeable at all.
To request a change, open the booking under Trips and use the change request option. Tell us the new dates you want and we will come back with the exact cost before anything is charged.