At the moment there is no official W10 available, update is planned (free of charge). A version of W10 can be obtained via the Insider Programme, which works very well on the L640.

Windows 10 is included free of charge and the pre-release version works flawlessly so far.

The Windows 10 mobile update can be carried out, I have done so.

There is no Windows 10 for Lumia 640 so far. Whether it will come is questionable, as the new Lumia lines x50 all come with Windows 10.