Look along the top row of keys, which should be F1 through to F12. Two of them will probably have 'Darken' and 'Brighten' symbols on the. In the illustration here they're on F1 and F2
https://www.computerhope.com/issues/pictures/ch000594-fn-brightness-2.jpg
but on the laptop I'm using they're on the F5 and F6 keys instead. Press the 'Darken' key and the 'Fn' key simultaneously to dim the display.
Occasionally the 'Darken' and 'Brighten' keys are found elsewhere on a keyboard, such as here
https://www.computerhope.com/issues/pictures/ch000594-fn-brightness-1.jpg
but the method is still the same. (i.e. locate the 'Darken' key and press it together with the 'Fn' key)