If you got your plugin from marketplace, it’s most likely has been installed on the Engine level, and you can find it in Engine\Plugins\Marketplace\. And usually those plugins come with the source code (since you editing it already, then you have it)
What you can do is to copy the plugin folder (the folder with .uplugin file) from Engine’ aforementioned location to your ProjectFolder\Plugins. This way starting the project will rebuild the plugin with changes you’ve made.
Hope this helps 