@Gary_Berg,

The only way to manually trigger the update appears to be through the general firmware update screen. This may have something to do with the way that the plugs communicate with each other in the home, as two devices running different firmware may not function in a group correctly.

The only workaround that I can think of is that when you first set up a device on your app, the device checks for a firmware update and offers the update. This may only be for one device, and you may be able able to trigger the prompt by performing a soft reset and connecting the device to your network again, but I cannot be sure that this will not try to update the rest of the devices.