Both... You can update while plugged in. And the infotainment lives on the 12v system. I wouldn't worry about the 12v dying unless you've been letting it sit for a week.
If you want to be on the extra safe side, drive it around a bit today, then start the update tonight once you've plugged it in.

 

In my opinion, a better option is to use Utility mode. The infotainment system will be inoperable while updating, so driving might not be optimal.

Utility mode uses the traction battery, also referred to as the high voltage battery, not the 12V. Then you can do the driver’s window dance, lock the car and walk away for the time it takes. The car won’t be drivable in utility mode. Read up on it in the manual or search the forum for "utility mode" to find out more.

See here to do the dance.

How can you lock the car while it’s running and there’s...

For example, let’s say I am the driver and my wife is in the backseat with my 6 month old daughter, how can I leave them locked in the car with the AC running and the car on? Right now the only way is if my wife reaches over from the back and hits the lock button after I’ve left. There has to...

www.kiaevforums.com

 

I've learned to just put my vehicle in Utility Mode and then perform the update--all within the safe and secure confines of my home/garage.

 

Does the sw update complete faster when the car is left in utility mode vs being left turned off altogether?

 

On mine the yellow light is on in garage but as soon as I start the car light goes off. Gives me ithe impression the high voltage battery does not charge the 12 volt while driving.

 

The light isn't there to tell you it's charging the 12v battery. It's there to warn you that the high voltage system is active at a time you wouldn't otherwise wouldn't expect it to be on. It's just that charging the 12v is the only reason it turns on unexpectedly.