Does anyone know a method for unlocking used dll in the unity editor? I'm developing a c++ dll which i'm including in my projects "Plugins" folder, but every time I want to update my c++ dll in my project, I have to close the editor before I can delete the old and copy in a new.

As other mentioned, this is not possible right now, and likely won't be any time soon, since we don't know if the dll is in a state where it can be unloaded without leaking anything (though it would be possible to expose mechanisms for that).

What you could do though would be to implement this yourself by creating a "wrapper" dll which loads and unloads the actual dll you want to use.