Override controller module

[cycy]

Hello, I wish I could change the controller of a module, however, via the theme it does not work and override either. I must surely be wrong somewhere. So I have a module / controller / front / ajax.php and it is this document that I have to modify. I tested via the theme but no changes are visible. Via the override also but nothing is visible. Can someone help me ?? I block on it for several hours, it disturbs me because I do not want to touch the original code (to avoid any problem with any future update).
Thank you very much, any help is welcome