Steps for "Update" operation using Data Loader: in Salesforce

Update --->This operation is used for modifying the existing records.

Note: For updating the existing records, we need Salesforce record ID(15 digits or 18 digits)

1. Double click on Data Loader icon at your desktop and Click on "Update" operation for loading the
records into salesforce.com

Step 1 : Salesforce Settings
Step 2 : Select Salesforce object and csv file
Step 3 : Mapping
Step 4 : selecting the directory