Please see http://www.united.com/page/article/0,8566,1141,00.html:

3) What you can do with a United award ticket

a) Book a ticket for someone else, even if they’re not traveling with you.

b) Stand by for an earlier or later domestic flight departing on your original date of travel and flying to and from the same places as on your original ticket. This option is only for flights within the U.S., and there may be a fee. Learn more about same-day travel changes.

c) Request a same-day change to another United or United Express domestic flight within three hours of the time of your request. There may be a fee. Learn more about confirmed same-day travel changes.