You can create a custom theme for your application. This will let you make changes to the layout of your application without changing the standard Atlas Theme.
https://docs.mendix.com/howto/front-end/customize-styling-new/#create-theme-mod
In your case, I would suggest looking at the current page's layout in your browser's developer tools, and find the bit of CSS that is centering the menu. Now write an override for that CSS selector and change the positioning there. Save this to your new custom theme module.
Good luck!
Hello Ozge Catak,
You need to create a page layout and structure your page with the navigation bar at the left.
https://docs.mendix.com/refguide/layout/
In the atlas UI module you already find some examples and maybe you are already using one.
Hope this helps,
Good luck!