This recipe works for:
SharePoint 2010 Foundation
SharePoint 2010 Standard Edition
SharePoint 2010 Enterprise Edition
Office 365 (SharePoint Online)
You will need a SharePoint 2010 Team Site or any other SharePoint site to which a tasks list has been added.
Open your Team Site. Select the Tasks link from the quick-launch menu.
A list of the current tasks for the Team Site is displayed. Select the List tab from the List Tools ribbon.
Click on the Datasheet View icon in the ribbon to open the datasheet view of the Tasks List. You can now perform bulk update operations to quickly update multiple items.