trusted number

Go to http://appleid.apple.com, from : Two-factor authentication for Apple ID - Apple Support

You can update your trusted phone numbers when you follow these steps:

1. Go to your Apple ID account page.
2. Sign in with your Apple ID.
3. Go to the Security section and click Edit.

If you want to add a phone number, click Add a Trusted Phone Number and enter the phone number. Choose to verify the number with a text message or automated phone call, and click Continue. To remove a trusted phone number, click next to the phone number you want to remove.

I dunno how to off the two factor authentication.. i have been log in into my account for a number of time yet the two factor authentication cannot be turn off as its like being locked somewhere.. please advise

From the page that I linked to :

You can't turn off two-factor authentication for some accounts created in iOS 10.3 or macOS Sierra 10.12.4 and later. If you created your Apple ID in an earlier version of iOS or macOS, you can turn off two-factor authentication.

Keep in mind that when you turn off two-factor authentication, your account is protected only with your password and security questions.

To turn off two-factor authentication, sign in to your Apple ID account page and click Edit in the Security section. Then click Turn Off Two-Factor Authentication. After you create new security questions and verify your date of birth, two-factor authentication will be turned off. If you don't see the option to turn off two-factor authentication on your Apple ID account page, that means you can't turn it off.

If someone has turned on two-factor authentication for your Apple ID without your permission, you can also turn it off from the enrollment confirmation email sent to your Apple ID or rescue email address. Click Turn Off Two-Factor Authentication at the bottom of the email to restore your Apple ID to its previous security settings and regain control of your account. The link is active for two weeks after you enroll.