To debug an existing Zebble plugin (or Zebble core itself) follow these steps:
The default location is at %userprofile%\.nuget\packages
For UWP, if in your local nuget cache folder, you see runtimes and ref folders, replace the dll in both ref and runtimes/...86 folders. If it didn't work, try the ..64x bit folder as well.
If you have multiple versions, use the latest version, or the one referenced in your app project.
Build your app. It will automatically get the updated DLL from the nuget cache folder.
In Visual Studio (loading your app, not the plugin) open the source file in Zebble/plugin and attach a breakpoint.
Run, as normal, in debug mode.
If you faced any problems, check the bin folder in your app to ensure the DLL is updated. Check the dll file date to make sure.