You can do it in your Personal Area in the "Profile settings" section (available after clicking on your avatar at the top of the page).
Please, scroll the page down until you see the "Security" section. In the "Confirmation method" line, click on the "Email"/"SMS" button. In the opened page choose the desired confirmation method, enter the code received via e-mail or SMS and click on "Confirm".
You can choose SMS as your confirmation method only in case your mobile phone number is verified.