If you want to change the mobile number on your WhatApp account without loss your groups/friends and others data. You can easily change your number with the latest update(s) on WhatApp for all platform.
You are almost near to your target, Click on Next (available to right top corner on screen).
Now, Enter the the old phone number & new phone number.
This will move your account info, settings and groups to your new number. But bad is that, there is no any option for change the number on Nokia Symbian and Windows phone yet 🙁
Remember: You should have both phone, old and new with you in active mode at the time of this process.
Enjoy… Happy Chatting… 🙂