How to change your resolution ingame
This is pretty easy, just open your options and change them in the upper-left corner.
How to change your resolution in the configs
Close the game
Open your GameUserSettings.ini within your Steam/ARK:Survival Evolved gamefolder (typically: C:\Program Files (x86)\Steam\steamapps\common\ARK\ShooterGame\Saved\Config\WindowsNoEditor) and change the two entries below as you wish.
Start the game again.