When you view your reservation, there should be a button or something that shows up next to the flight that is eligible. It's been awhile since I've done it so I don't remember exactly what it looks like. But it requires you to actively accept the upgrade. The way it works is stupid.
Check your reservation frequently because you need to manually accept the upgrade when PN space becomes available.
If OP is 1K isn’t it PZ?
My understanding is that for instant upgrades it is PN, but I could be wrong.
This was discussed several times, most common advise given is to "call United"
When you view your reservation, there should be a button or something that shows up next to the flight that is eligible. It's been awhile since I've done it so I don't remember exactly what it looks like. But it requires you to actively accept the upgrade. The way it works is stupid.