3840 × 2160

My LG 27GL650F-B 27 monitor is stuck at 3840 × 2160. Everytime I open a game is says "your base resolution is 3840 × 2160 switch to the recommended 1920 x 1080" but i dont know how to change it, this is causing my pc to lag and games to be stuck at 30hz unless i go into windowed/borderless mode...