It is not possible to change your Chamber of Commerce registration number. A Mollie account is always tied to a specific company, with a specific Chamber of Commerce registration number.
When you opened the account, you stated on behalf of your company that you agree with our Terms of Service. If you want to avail yourself of our services for a different company, you will need a new account and once again agree to our Terms of Service.
Go for more information about setting up a new Mollie account to How do I sign up for Mollie?