United Airlines may charge a fee if you need to change your reservation. The fare type you purchased and the changes you require will determine the cost of the fare change. Changing United flight – a Basic Economy or non-refundable Economy fare will cost you the difference in price and a change fee. You may be able to make free changes if you bought a refundable Economy or Premium cabin fare.
Source: https://flyustravels.com/blog/changing-united-flight/