The FAQ covers this in some detail - I'd suggest reading that first.

What you're doing here is called a same day change (SDC). You can change to another flight, starting 24hrs before the original flight you're on, and move to any flight within 24 hours of the current time.

If inventory is available in your original booking class you can confirm this. This can be done online or over the phone. If it's not available, you can go to the airport within the 24hr period and be put on standby on any flight subject to the same time constraints above.

So if your original flight was Sunday morning, starting on Saturday morning at the same time you can either get confirmed on, or go to the airport and standby for, any flight between then and the same time the next day.

Hope this helps.