Wp Modifications (no themes) and updates?

Hi guys,
I’m trying to search an aswer for my question but still don’t find it. I’ve made some modifications on my wordpress php files, so not on themes, but on wordpress, like for example in the wp-login.php to edit the theme of login page and on a plugin css inside the plugins folder. Then I’ve updated wordpress and the plugin and all my modification disappear. Fortunately I had a backup for them, but there isn’t a way to make wordpress not to update those files modified, like for the theme child?