First, you should remove the old one.
See How to Remove Phone Number From Facebook
http://www.youtube.com/watch?v=FozfP8QjDtI
After removing it , click on "+ Add another mobile phone number" and follow provided steps.
Open your facebook account then click on edit profile and change mobile number
Welcome to Facebook Log In Sign Up or Learn More and login then open account setting change mobile number .
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If you want to change you mobile number in your Facebook account so open account setting and change contact number .
Change your phone number in your Facebook account Settings, General Settings.
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