We always fly United. It's really our only choice where we live. Anyway....when choosing your flight on United.com (even if you initially choose economy/coach), you will get to the seat selection part. It's usually color coded and when moving the mouse around will specific coach or economy plus and when mouse over economy plus, it will show you the cost extra PER SEAT. If you don't choose economy plus during your seat selection, it always asks you before you finalize if you want to upgrade at "x" amount. The cost is dependent on what you're comfortable paying. My husband is tall and he feels cramped in coach but sometimes the cost of upgrading to economy plus is too high (for us). Once, and only once did this happen to us, our flight times got changed and there was a voicemail to call United. When I did, they gave me our new flight times and then offered to upgrade to economy plus which was very reasonable. Sometimes it can run $50 plus per seat to upgrade and they offered $80 for the three of us. I couldn't believe it. I think that doesn't happen very often though.