We were given a huge dieffenbachia when my father in law died. My husband realized it had been in the house for a long time and set it outside to get some sun. Yes, this was a bad idea! What can we do to quickly remedy the “sunburn” the plant got?
Hopefully, you put it back into the house when you saw the leaves starting to burn. There's nothing you can do to "save" the damaged leaves. When plant tissues are dead, they're dead. However, the plant itself should be okay, and it time will put out new leaves, if cared for properly. Cut off the damaged leaves at the point where the leaf stem is attached to the main stem. Keep the plant in bright indirect light in your house - anyplace that you can read comfortably. If you want to put the plant outside for R&R, it needs to be in heavy shade. Indoors or out, you need to be careful to not overwater - the soil in the bottom of the pot should be very lightly damp before you water again. A moisture meter that you can get in any plant store or dept will help you alot.
Here's more information on dieffenbachia: https://www.gardeningknowhow.com/houseplants/dumb-cane/growing-dumbcane-dieffenbachia.htm