When you update while converting a lead, you can update specific records. Not every record can be updated by the system. Certain types that can be updated when converting a lead include the existing opportunity record and the existing account record. These records have basic fundamentals incorporated in the system which is why they can accessed for further storage.
The Administrator can exploit the content and save new information as desired. It is important to take note that only these records can be modified. Others like the existing contact record or the existing custom object record cannot be updated while converting a lead.