Need to change phone number

You can edit your personal info here, including your phone number: -

https://account.microsoft.com/billing/addresses?ref=amcyourinfo#/

You may also need to change your phone number on the relevant security page if you're using a phone number to receive security alerts. Go here.....

https://account.microsoft.com/security

.....then click the blue "Update info" button and you can add/remove phone numbers there.

Sorry to hear it. You can change your personal information at
https://account.microsoft.com and any other contact preferences can be changed here https://account.xbox.com/ContactPreferences

Has your query been resolved ?

I would like to change my old phone number to my new number

You can change your security info like phone numbers and emails at
https://account.live.com/Proofs/Manage.

However, unless you have means to verify your old phone number (or other security proofs you already added), it will take 30 days to change your phone number.

This is to prevent hackers who got your password to add their own phone number or email without prior verification through the existing security proofs.

Also, while on that page, if you haven't already done so, you should enable Two Step Verification and set up the free Authenticator App on your smartphone as well, to make your account hacking proof.