A passenger needs to do the update to add an extra equipment, change the class, special seat but you don't know the process of doing the update. Then below you can see how to upgrade on United airlines.

https://www.lotofares.com/es/noticias/como-actualizar-en-united-airlines

The process of making the update on United airlines

A person can update their bill using the internet. Adelante una persona puede ver los pasos de actualizar el boleto en las airlines de United.

Los pasos to update

First of all, a passenger can go to the official website of United Airlines.
On the main page, you need to open the “Mis travels” option.
In this section, you have to write the confirmation number and last name.
After that, use the “Search” button.
On the next page, you can see your flight booking.
Now you need to update your ticket.
Finally, a traveler needs to pay the cost of updating their credit or debit cards.
Note

A traveler will receive the confirmation of the update in the passenger's mobile number and in the electronic mail.
You cannot perform the update after billing or after receiving the boarding pass.

On this page you are able to see how to upgrade on United Airlines? . If you cannot make the update on United airlines then you should make a customer service call. You can find the official number on the official website of United Airlines.