cant change my phone number

What exactly is happening when you try to change your number?

Has your query been resolved ?

Hi. My name is Brilliant and I am happy to assist you today.

To remove / add / update a phone number on your Microsoft account:
1. Go to "Manage how you sign in to Microsoft" and sign in to your account: https://account.live.com/names/Manage

2. Select "Add phone number" and follow through the process of verifying your contact details and adding it to your account.

3. You can make the new contact the primary alias by selecting "Make primary".

4. You’ll then have the option to remove the old email address or phone number from your account.

Hope this helps resolve your query. Have a great day.

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.