Update:
1. Double Click on the icon data loader present on the desktop.
2. Data loader will opens, then select operation what do you want to do with data loader.Click on Update button.
First you need to know the what is the exact function of update. Update means already inserted records you have to change something on it.It's means changing phone number or change address like that.
Before updating you need to have the record ID's of inserted records.This will get you in two methods,
one for inserting records in to object one success file will generate. in this success file having all the records id and details.
Second one is Using data loader to Export the records from which object you want.The following steps explains updating records in to salesforce using apex data loader.1. Double Click on the icon data loader present on the desktop.
2. Data loader will opens, then select operation what do you want to do with data loader.Click on Update button.
3. Then same process we done before in insert operation Enter an Email id and Password with Security token click on login.
4. Select an object to update and Updated .csv file also.
5. Click on next it will initialize, and it's says your operation contain some records.Click on ok. Then Choose a mapping.There is two options one is Choose Existing mapping and Create or edit map.choose existing mapping is you inserting time you saved map file you can select it and second one is Create or edit is you can create new map. Same in insert operation but here one new field comes in to the picture. that's ID.
you must and should map with objects ID.
6. Drag and drop process to complete the mapping.then select Next button.It's says one popup window warning You have chosen update operation. Click on proceed.That's it.
7.It will generate success file and error file.open the success file then see your modified records.
