WhatsApp offers you the option to change the phone number verified in WhatsApp from within the application.
Learn more about changing your number: Android | iPhone | Windows Phone | Nokia S40 | BlackBerry | Nokia S60 | BlackBerry 10
Note: Make sure to use the specific Change Number steps to migrate your account to your new number. If you simply verify a new number with WhatsApp (you might see a prompt to do this when you put a new SIM card in your phone and open WhatsApp), your old account data will not migrate over to the new account.
Please do not swap SIM cards or phones frequently. This could create problems in the verification process. Read this article for more information.
Cheers,
WhatsApp Support Team