To select the Player you want to play, go to the Player section and select the Players with the checkboxes. The Shortcuts can be changed by clicking on the button for the action you want to change and then pressing the key you want to use. The changes are loaded by clicking on  OK or Apply. The current game will be ended and a new one started. You can click on the  Defaults button to restore the default Shortcuts.

A Granatier Config screenshot