Hi @Christopher-Xavier, and welcome to Auth0 Community!
In your Auth0 Dashboard you can go into User Management > Users then search for your user and click the three dots (…) to select the View Details option. Here, in the first tab, Details, scroll down to the Multi-Factor Authentication section and click the Reset MFA button. This should clear out all enrolled MFA options for the user so that you can register a new phone number.
I hope this helps!
Teodor.