MFA Access to Salesforce and lost/replacement device

MFA (multi factor authentication) is typically managed by users installing the Salesforce Authenticator mobile application.

Here is a Salesforce article on how to install and configure the mobile app:

https://help.salesforce.com/s/articleView?id=sf.salesforce_authenticator_add_account.htm&type=5

User has lost phone

If the user has lost their phone and needs to setup a new phone, the quickest way is for the Salesforce Admin to remove the MFA device in the User profile: