On the major airlines, if you are not going to use the return portion of your ticket on the date you are scheduled, call the airline and cancel it. On a restricted ticket, the unused portion can then only be used on the same routing typically within 30 days from the outbound flight after paying a $75 change fee and any fare difference if the fare class in which you were booked is not available. If that fare class is available on your re-scheduled flight, then you will be asked to pay a $75 change fee. Some tickets are even more fully restricted (special promotional fares like e-saver fares and others) so your unused return portion may have no value.

On Southwest Airlines, again, call the airline to cancel any reservation you will not use, and then the value of the unused portion can be applied to any Southwest routing up to 365 days from when the original ticket was purchased.

Other discount carriers have different rules, so you should just call the particular airline.