How to change registered phone number for my on line HMRC account -

I have tried for a week to speak to an advisor without success and the information received via the on line support based on a link is not valid - the link does not exist and I am going round in circles . I have lost my password and cannot reset as the verification code gets sent to my old mobile - HELP

Hi,

If you are unable to change this online, you will need to contact our online services helpdesk to remove the old number the code is going to.

Technical support with HMRC online services

Thank you.