I asked about this a few months ago. If you recompile your game module in visual studio the editor detects the new hot reloadable DLL and reloads in place. For plugins however there is no such file watching. You can go to dev tools -> modules and manually recompile but it is a bit annoying. I asked if the file watch could be implemented since there isn’t really any reason why it would not work, a bunch of logic in the HotReload code needs to be updated to also search plugin binaries folders.
https://answers.unrealengine.com/questions/493850/hot-reload-of-game-plugins.html
Waiting on UECORE-343 (it is not in 4.15)