Navigate to the "User Details" page, click on "Edit User" and then enter the user's new phone number in the phone number field.
You will then ask the user to navigate to the sign-in page and begin signing in. They will be prompted to request a code via text or call. When receiving that code, they will select "Send again Wrong number?"
The user will get a "Passcode" that you as an Admin, will enter under their new phone number. After this is done, you will be able to save the new phone number.
For any additional information, please contact your Customer Success Manager or reach us at firstname.lastname@example.org.