Probably the easiest will be to delete the folder from the plugins directory. After launching Gramps, go to Addon Management in Preferences and reinstall the most recent version of the addon. You might have to uncheck the Do not ask about previously notified addons.
This is part of the core Gramps program files.
The plugin folder is (on Win10)
c:\user\%you%\appdata\roaming\gramps\gramps51\plugins
FYI: The appdata folder by default is hidden.