Currently this option is disabled to avoid fake contact details. Therefore, we advise you to please enter your correct phone number at the time of registering your account.
And if it is really necessary to change the phone number, you can request us through an email at firstname.lastname@example.org. Please don't forget to mention your member id, your old and new phone number. If you will be failed to provide aforementioned detail, we regret to tell you that we will not be able to change phone number from your account.