If both your security contact details have changed i.e. you have a new cell number and a new email address, you will need to contact the SARS Contact Centre or go to a SARS branch office as authentication (section 69 of the Tax Administration Act) is required. The SARS agent will change the details on your behalf. This does not apply to updating your registered contact details. See How do I update my registered details on eFiling for the latter.