This is an important best practice – never, ever modify plugin files directly.
Rule 1: there is never a good reason to do this.
Rule 2: refer to rule 1.
Note that the WP-Members plugin is full of action and filter hooks that allow you to build proper customizations for your deployment. This is the same way that WordPress itself allows you to modify the behavior of WP without touching the core of WP itself. There are also a number of API functions that you can use.
This allows you to store your customizations outside the plugin. The place that you should do this is in your child theme’s functions.php. If you are not using a child theme, you should be, but if you can’t then use your theme’s functions.php file.
An alternative method to using your theme’s functions.php file is to create a plugin file to store your custom functions. The advantage to this is that it is portable if you change themes. Using your theme’s functions.php file would require moving your custom functions to your new theme, but if you set up a custom functions plugin, then your custom functions are independent of your them.
Unless it is a patch for an issue with the plugin, nothing on this site will EVER direct you to modify the plugin files directly. So don’t do it.
When you modify the plugin’s core files you run the risk of breaking the plugin and possibly your WordPress installation. In addition, it makes it impossible for me to provide support for you, since I have no knowledge of what you’ve changed.