1. Once logged in to your account, click the 'Menu' icon at the top page's left corner and go to "Settings".
2. Select the "Security" tab.
3. Enable "Alternative 2FA" by clicking over it.
4. Type your phone number and click on "Send Verification Code".
5. The code will be sent to your mobile as an SMS. Enter the code in the boxes. Note that the code expires in 180 seconds. If the code has expired, you can request a new one.
6. All done. Your new phone number is now verified.
IMPORTANT
In order to verify a new phone number, your old number must be removed. If you need a phone number to be removed, please see the instructions on how to request it, here.