You neither earn nor can use US upgrades on UA. Your only option is to buy upgrades upon check-in at the airport. Web check in does nothing for upgrading--it's when your body is in front of them. You can not call in and be upgraded in advance either.

I'm flying UA for US miles for the first time soon and the advice from UA res agent was to call the night prior or earlier in the day of your flight to see if there's anything open in F to determine how early you want to get there.

Obviously, if all seats are taken, you needn't arrive terribly early in hopes of buying some upgrades.

I'm currently in E+ for my flight, hoping to upgrade. If you go to seatguru, some planes have an E+ seat (row 10 I believe) with no seat at all in front of it for the emer exit row! On other threads, E+ has gotten high praise--I'm curious and anxious to try.

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--Wishing for you safe travels and for me Row 1.