Shop for your flight as you normally would. Once you get to checkout, all unused travel credits that are connected to your MileagePlus account will automatically appear once you select “Travel Credits” as a payment option. All you have to do is select the checkbox next to the credit and it will automatically be applied to your reservation.

If you don’t see one of your travel credits listed, or don’t have a MileagePlus account, you can also look up your credit directly from checkout and apply it to your cart.

Who can actually use a travel credit varies based on credit type.

Future flight credits
Normally, a future flight credit is non-transferrable and therefore must be used by the same traveler it was issued to. However, all future flight credits that were issued for tickets purchased on or before August 31, 2021, can now be used to book travel for anyone, including yourself, friends, or family. If you have trouble using a future flight credit, please contact Reservations at 1-800-864-8331.

Travel certificates
These credits can be used by anyone.

To use travel credits with PlusPoints, you’ll need to book your ticket using your travel credit and then request a PlusPoints upgrade.

All travel credits do expire, but the expirations work differently for the two types of credits.

Future flight credits
Future flight credits have a “travel by” date as their expiration. This means the itinerary you apply a future flight credit toward must begin travel before the expiration date. In most cases, this date is one year after your original ticket was issued.

Please note, future flight credits issued on or before December 31, 2022, have had their “travel by” date extended to December 31, 2023.

Travel certificates
Travel certificates have a “book by” date as their expiration. Meaning, you must book your flight using this credit before the expiration date. In most cases, these credits expire up to one year after the date they were issued.

Please note, travel certificates set to expire on or before December 31, 2022, or issued before December 31, 2022, have had their “book by” date extended to December 31, 2023.

MileagePlus members can find a summary of their travel credits in their account. If you aren’t a member or can’t find your credit in your account, you can also use the lookup forms provided on this page to find your travel credit information, including the value and expiration date.

Yes, you can use multiple credits of the same type in a single purchase. Future flight credits and travel certificates cannot be combined. {Although in Sept 2021 UA hinted this was coming}

You can still use your credit, but you’ll need to do it through the travel agency you used to book the original flight.

Originally Posted by wiki

If you want to extend an FFC, the most reliable process is:

Purchasie a new non-refundable (non-BE) itin, for same passengers as the FFC. Do not use change of flight, start a new booking without using the FFC.
When you get to the payment page, then use the FFC as a form of payment
Wait a bit over 24 hours and then cancel.
You then will receive, essentially instantaneous, a new FFC with an expiration 1 year from the booking date.
The new FFC must be used for a trip starting before the new expiration
Deviations may not be successful
This will not work for FFC from a partial flown trip.

This works for extending ETC also except the ETC with be converted to a FFC and locked to the named passenger.

There is no guarantee this process will work in the future