Currently, AFAIK, the best way to do it is (on desktop app) to open the note in an external editor, and find/replace
[ ]. On the mobile app there's no way to do it, you have to uncheck each checkbox individually.
An « Uncheck all » command would be most welcome !
IMO such a functionality should be natively implemented in Joplin, not through a plugin, because it could be used a lot from the mobile app (I am thinking of things like a shopping list, etc.).