To reset MFA for a user in Microsoft 365, an admin signs into the Microsoft 365 admin center or Entra admin center (Azure AD), finds the user, navigates to their authentication settings (often under Users > Active Users > Manage multi-factor authentication or Identity > Users > All users > Authentication Methods), and selects the option to"Require re-register multifactor authentication," which prompts the user to set up their methods again on next login