You have to remove it by using a flat pry bar, most have a notch just for that. If it's a roofing nail, you should be able to get it under the head and pull. you can find the flat pry bars in small med. and large.
Another possibility is if you have a start already, you can get the flat pry bar behind the sheet at the nail and lift it out from the insides but for this, you have to be accurate in finding the nail or you will have a big hole in the panel. If you can raise it @ 1/4 inch, you can then grab it on the outside with a claw hammer and remove it.
Another possibility is to take a small grinder and grind it down in the center and popping the head off the nail and pull the panel off the shank of the nail.
Do not use a nail puller,. it will do even more damage
Next time,. hang it with 4p finish/casing or paneling nails, this will allow you to remove the paneling by letting the nails be pulled thru the back and leave only a small hole on the back.
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