First, open the file where your game is installed. For example, C:\Program Files (x86)\GameSessions\Overlord II.
Run the Config tool. Here you can change various settings, including the resolution. Change what you want, then click on "Confirm and Play".
The game launches, and asks you if you wish to keep these changes. Accept the changes to proceed with the game. Note that if you reject the changes or the timer runs out, the game bugs and displays a black screen. Simply quit the game by pressing ALT+F4 and restart it to fix this.