Lumia 638 is denim updatable I have just done this if you go to windows insider program app Then you phone first update to lumia denim if your running lumia cyan then you will be able toupdate to windows 10.
But some says microsoft website. windows 10 mobile update is not available for lumia 638 alone in india. when I enquired about this issue to microsoft, they said that denim update will be launched officially for 638 then onl our phone can be updated to windows 10.