Resetting a user's MFA details requires the user to re-register at next log-on. Proceed as follows.
Go to https://portal.azure.com, and sign into the Microsoft Azure portal using an account with administrative privileges.
From the left-hand menu, click Azure Active Directory and, from the options given, click Users.
A list of All users appears.
Search the list for, and click on, the desired user.
The selected user's Profile page appears.
On the left-hand menu panel, under Manage, click Authentication methods.
An option bar appears at the top of your screen.
Click Require re-register MFA.
If the user's device has been lost or stolen, then also click Revoke MFA sessions.
This resets the user's MFA details, and they must now re-register their MFA methods upon their next sign-in