From security & performance perspective it is recommended to keep your WordPress, plugins & themes up-to-date. If you want to make any customization in a theme then you shall develop a child theme. If you want to make any changes in a plugin then you should use hooks & filters in the plugin and should not edit core files of the plugin.
However, sometimes you may want to use a plugin or theme and then customize it to a greater extent and you are ready to maintain it on your own. In this case you would want to suppress the plugin update warnings that shows up on plugin listing page on WordPress dashboard.
To do that, you need to locate the main php file of the plugin where plugin headers are defined. Once you locate that file, open that in any code editor and bump the plugin version to somewhat large number. In below image I’ve shown how I suppressed “Admin Menu Editor” plugin updates by editing the plugin version to larger number.
In case above GIF image does not work out for you then see below image that highlights the code file of the plugin and line which I edited.
If you are developing a new plugin by using someone else’s plugin as a starting point, then it is good idea to fork the original plugin and start your own. Never forget giving due credit to original plugin author in your forked version of the plugin 🙂