There are a few different makes of PC in the library. Some of them will have brightness control buttons on the edge of the screen, either at the bottom or the left hand-side.

Once you are logged in to a PC you can also access the brightness controls on the Action Centre icon at the bottom right-hand side of the desktop (step 1 - highlighted with a red box in the image below). You may have to click Expand to see the brightness level (Step 2)

Use the slider at the bottom to adjust the brightness:

There is also a tile for 'Night light' which will dim your screen.