Since the game doesn't have a sufficient way to change the settings in the game you'll have to edit the config file yourself.

To do so simply navigate to your config file, which should be found in this folder:
C:\Users\User\AppData\Local\FPSChess\Saved\Config\WindowsNoEditor

And look for a file called GameUserSettings.ini ,open it with the editor of your choice eg. Notepad. (right click > open with > notepad)

Adjust the settings in the ScalabilityGroups to your likening, for example this is what I'm using to run this game at a stable 60fps on SteamDeck:

(default Value for everything but the Resolution Quality is 3)

Then save the file and restart your game.