If their phone number has changed or are using a different phone, and they do not have their backup codes, we can reset by going to Setup > Other Setup > User Access Reset Tool (or can search for the page)
Type their email address in, tick the box Reset 2FA settings and then click Save.
The user will need to verify their login with a code sent to their email, then they will be shown the QR code to add to their Authenticator app.