Updating accounts from an import based on an unique account id

Updating accounts from an import based on an unique account id

I have a legacy system (no api available) which has active clients in it.  The system is really the master for the client list and is used in the company's operation.  If a client changes address it is changed in the legacy system.  Each client has a unique ID which I have created a field in Zoho CRM to capture it. 

On the first import of clients I can import them with the ID but is there a way to update them on future imports.  I don't want to rely on contact name or company name because if it is changed then on import it would not match.  Is there a way to import and match on a unique ID number from the other system to update existing contacts in Zoho CRM.  Also on the import if there is no match (new client) add to Zoho CRM.

Thanks in advance for the help.