how to change two factor aunthentication number

You need to change/add the number on your Apple ID. You may have to add the new number before you can remove the old number. After the change, allow up to 24 hours for the change to sync. Before trying to make the change, sign out of Messages/FaceTime on all devices/computers.

Apple ID - Manage 

From the article below.

Update your trusted phone number on your Apple ID account page

1. Go to appleid.apple.com.
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.

Apple ID -Two-factor authentication    

Read this document:

Two-factor authentication for Apple ID - Apple Support

Note that you can set multiple numbers to receive your 2FA codes.

You can even receive the codes via “voice-only” phone calls.

Give some serious thought as to how you’ll regain access to your AppleID in a worst case scenario. (generally a home fire / flood / storm and physical loss of ALL trusted Apple devices)

I am trying to set up wassap on my phone, I had it before and now put my chip in a different phone, it’s telling me it will send a sms code, i am not getting the code, it’s just asking for my number, I cannot remember my two factor authorisation code