Same day change option missing
 

I'm flying tomorrow at 4:30pm and checked in a few hours ago. I just returned to united.com to see if there were any earlier options and the button in the check in area is missing. I remember being able to change flights after checking in before, does the button disappear if there are no options available or am I unable to change after checking in?

Yes, it does disappear if there are no options available.

Remember, you can only change to flights that have inventory available in your purchased fare bucket. This is obviously more of a problem if you are on a cheaper ticket. That said, UA tends to open up all fare buckets within 3 hours of departure, so check again closer to the flight you want to take.

If you have expert flyer, you can set an alert for the flight(s) you want to take and have it look for availability in that class.

Same day change option missing
 

I have better luck going on united.com and choosing change flight option.. Some times for 20 dollars you can change flights. I also get free options the App on phone don't show.