You can change the mobile number used in your WhatsApp account, if you need to access the same account including, chats, groups etc with new mobile number. This can be easily done inside the WhatsApp itself.
First open your Whatsapp and then click on the menu. From there select Settings
In the settings page, again select Account. In the Account Page, you can see an option called Change Number. Click on it.
Now you will see a new page with some message saying
‘Changing your phone number will migrate your account info, groups and settings. Before proceeding, please confirm that you are able to receive SMS or call at your new number. If you have both a new phone and a new number, first change your number on your old phone.’
This is only an instruction, as for changing you have to provide the new mobile number so, it should be able to receive SMS or Call, since we need to get the OTP to verify the new number.
Also, as per the last instruction, if you are going to use your old WhatsApp in a new mobile with new number, first you must do the number change option in your old mobile and do take the necessary backup of the chats. After that you can install WhatsApp on new mobile and register using the updated new mobile number, so you will get all the chat, group and other account on your new mobile.
Ok, After reading the instruction, you need to click on the Next button.
In the new window, you need to provide your existing WhatsApp mobile number and the new mobile number. Then click on Done.
Now you will get a OTP on your new mobile number. Enter the OTP in the next window. Then click Submit.
Done, now your WhatsApp account is updated with new mobile number.