How to downgrade from Lollipop to Kitkat? I am using Spice AndroidOne. When I purchased it, there was Kitkat in it. It automatically requested for Lollipop update which I have granted. Later it has requested for Marshmallow update. When I tried to update it, it has given an error. 

Now Android OS is eating all my data (3G) in one day without my permission.

How to rectify it. And also I want to go back to Kitkat version.

Kindly help me.
Unfortunately you cannot downgrade from the newest version of Android to an older version, unless you root or modify your device's permissions. Ask your carrier to see if they can flash a new ROM to your device.