How switch to the India store for whatsapp update? I do not want to change my number. i just want to update whatsapp?
If you originally downloaded WhatsApp from the Indian store, all you need to do is log into your Indian Apple ID account when prompted (after choosing to update WhatsApp). 

Once you log in, the App Store will prompt you that the ID only works with the Indian App Store and will then switch over to that store and you can do the update. It will then stay logged into that account until the next time you change it by logging into another account.

The store you download the app from has absolutely no bearing on the number etc associated with your WhatsApp account.