Usually there are keys that you press in conjunction with another key like the Fn key and a key that looks like a bright Sun, or a Dark Sun, shape, on one of the keys usually on the number keys on top row under Function keys. So you press the Fn key and at same time. tap the Bright sun key, and it will step up in brightness, plugging in the adapter also makes it brighter too.
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Usually there are keys that you press in conjunction with another key like the Fn key and a key that looks like a bright Sun, or a Dark Sun, shape, on one of the keys usually on the number keys on top row under Function keys. So you press the Fn key and at same time. tap the Bright sun key, and it will step up in brightness, plugging in the adapter also makes it brighter too.