If you want to change any keyboard settings on Debian, you have to install keyboard-configuration package. You can use this command for package installation:
sudo apt-get install keyboard-configuration
Keyboard settings stored in /etc/default/keyboard file. You can edit this file for changing any keyboard settings.
To change the keyboard layout, find the XKBLAYOUT line and edit the value.
After editing, you can restarting keyboard-setup service or OS to apply new keyboard layout.