To change your e-mail or mobile number follow these steps:
First you need to be logged into the SITS Registry
When logged in, click on the user menu in top right corner (it shows your name) and select Personal Preferences
On that page, under Security section, in Two-factor authentication table, click on "Edit" next to the type you want to change
In the box that appears, enter your new e-mail/mobile number
Press "Send code" button to send a verification code to the new e-mail/mobile number
Enter the received verification code in the input field below and press "Save"