how do i change my phone number in iphone x

Iphone number:

Settings>Phone>My Number... tap on it and edit.

If you're talking about the number where the two-factor authentications send their codes, read this support guide:

Two-factor authentication for Apple ID - Apple Support

If you get no help: Contact Apple Support using this link:

Contact Apple for help with Apple ID account security - Apple Support

Regards

Giulio