To adjust the brightness of your display, swipe downwards on your tablet. This will bring down your Notifications Drawer, which also contains some shortcuts to your Phone settings.
Under the row of setting icons, to the right of the drawer, there is a small drop-down arrow. Press this arrow to expand the settings section.
At the top of the drawer a bar will have appeared; this is your brightness setting. Swipe the icon along the bar to adjust the brightness of your display. The further to the left the brightness will decrease, the further to the right the brightness will increase.
Pressing anywhere on the screen (but not on the Notifications Drawer) will save your adjustments.
Alternatively, you can go into your Phone settings through the folder on your home screen, select the Display option and adjust your brightness level there.