Reset Office 365 MFA / 2FA due to a lost, stolen or replaced phone.
Open Office 365 Admin Portal https://portal.office.com/adminportal and click Active Users
Click Multi-factor authentication
Select the user you need to reset MFA / 2FA
Click Manager user settings
Click Require selected users to provide contact methods again then click save. The next time the users logs into their account they will be prompted for new MFA details.