Sorry to hear you're having trouble.
To change any personal information you would need to first log in to https://account.microsoft.com and then click on Your Info on the left side and finally scroll down to where it says edit account info. Please note that you may need to remove the phone number prior to adding the new one. Also please remember that changing information such as a phone number will put a temporary 30 day freeze on your account so you won't be able to add or purchase anything during that time.