I had 3 sets of AirPods. 2 upgraded inside of an hour by leaving them connected to my MacBook, while they were charging and the AirPod cases were open.

One didn't upgrade. Annoying.

I left it for about 4 hours.

Anyway, you can force an upgrade by hitting "forget this device" and then re-pairing it. As soon as you do, the firmware will upgrade.