If you are upgrading for the first time, you may have to follow a few steps. First, you have to request an upgrade. You will have to complete the United Airlines Business Class Seat Upgrade process, so make sure you follow each of the steps below and don't skip any of them.
In this way, you can do upgrade United Airlines flights and get the seat of your choice. If you face any issues while doing the upgrade, you can connect with the United Airlines support person.