Unable to change debit order details

It,s been almost a moth that I ve been trying to change my debit order details to a new company account, as I have sold my vehicle, every time I call , I get several people and everyone with a different story and no one will be able to help me as this is very urgent, no one takes ownership of the call. <br> the last person I spoke to ask me to send an email to fleetupdate .... but no reply.<br> what must I do to change the details.