How to over-write records?

All the master which are imported from import data wizard can be over-written also. Lets consider a situation to understand how over-writing works.

I need to change the Default Warehouse for few items.

1. Download template for importing item master for import data wizard.
2. Check "Download with data" .
3. Keep only 3 columns. ID, Item code (primary field) and Default Reserved Warehouse in spreadsheet.
   Note: ID is only required when over-writing data. ID will be same as Item Code.

4.  Put data and then save file in .csv format
5.  In import data wizard, check field: Over-write records with same name?
6.  Browse file the import