Account maintenance

Can someone send me a link to someone who can help with account maintenance.

The company I work for needs the information, because we haven't been able to track down the email address that belongs to a specific serial number, and they want to change the user linked to it.

However, all we've gotten so far is download links and a link to assign the serial number (but again, we don't know WHO the serial number is assigned to, there has been turnover over the past few years).

How can they figure out which serial numbers are linked to with email addresses, and change them?