Sign in to your Apple ID account page and edit / update your Apple ID details -> https://appleid.apple.com
- (Sidebar) Click Personal Information -> Reachable at -> then edit / update your trusted phone number.
See -> Two-factor authentication for Apple ID
Two-factor authentication for Apple ID - Apple Support
If you are expecting an SMS verification code, check your Messages.
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See this Apple article: If you forgot your Apple ID password
If you forgot your Apple ID password - Apple Support
Then see this Apple article: What to do after you change your Apple ID or password
What to do after you change your Apple ID or password - Apple Support
B T W:
You are sign in here . . . with your Apple ID & Password.
Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.
You can’t merge multiple Apple ID accounts.
What happens when you use the same Apple ID & Password that you use to sign in here?
See the following Apple articles
Sign in with your Apple ID -> Sign in with your Apple ID - Apple Support
Manage and use your Apple ID -> Manage and use your Apple ID – Apple Support (UK)
