If you are using Salesforce Enterprise Edition or Unlimited Edition, you can use Data Loader to import, update, delete or export data to/from any object in Salesforce.com
In this blog, I'll give tips on:
Login to sandbox instance
By default, after Data Loader installed, it is configure to login to Production instance. You need to change in Data Loader setting to point it to sandbox instance.
Goto Settings and find "Server host (clear to turn off)", type: https://test.salesforce.com
Update fields to null
If you update a field to null / blank, and you notice the data is stay the same (not updated to blank), you need to change in Data Loader setting.
Goto Settings and find "Insert null values" and tick this checkbox.