This answer applies to LG 27UK600 and 27UK650, which seem to differ only in the stand and the back panel that connects to it
Note "up" means also slightly inward because the button is mounted at an angle - take a look from below and behind.
Press up on the button at the center of the lower back edge of the screen to turn on the monitor and then up again to get the circular menu displayed at the bottom of the screen.
Press the button to the right to get to the settings menu
Press it to the right again to get to the brightness menu
Press up to select the brightness adjustment slider
Press left or right to adjust the slider as needed
Press up to finish brightness adjustment
Press left twice to get out of menu system
Enjoy your 4k display!