How can a slight, dry, invisible impurity on a laptop be purified: by wiping, and should the wiping be with a damp or dry cloth?

If the impurity on your hand is dry, it will not transfer to the device. However, if your hand is wet and you are certain of its transfer, then the device does not need to be purified except for carrying it during prayer. Wiping it is sufficient since it is a smooth object, especially if there is a concern of damaging it by washing. It is also important to avoid obsessive doubts, knowing that some scholars permit small amounts of impurity generally.