https://support.communitybrands.com/hc/en-us/community/posts/30757902974989--1-888-829-1467How-To-Change-Date-On-United-Airlines-flight-Ticket
After the 24-hour period, United Airlines Change Fees will be
$200 to change the date and time of their flights. If your itinerary includes travel within the United States and Canada, there will be no United change flight fee.