Step A. Select the "Account Settings" tab from the top-right corner.

Step B. Select the "Security" tab.

Step C. Under "Two-Factor Authentication" click the "Change" button.

Step D

1. Change your old number for the new mobile number. 

2. Enter your login password to confirm this action.

3. Click "Change".

Step E. Enter the PIN number that will be sent to the new mobile number and click "OK".