The DemandTools MassEffect module can be used to update existing records using an input file in any Salesforce object that supports updates. Records can be updated one table at a time (per pass) using the standard SOAP API or Bulk API.
When updating records a matching key is required to match the records from the input file to the existing records in the desired table. By default, the Salesforce record ID is set as the matching key but can be changed to any field in the object. Below is a quick tutorial that covers updating existing records using MassEffect.
More information on the MassEffect Update Operation can be found here.
