This recipe works for:
SharePoint 2010 Foundation
SharePoint 2010 Standard Edition
SharePoint 2010 Enterprise Edition
Office 365 (SharePoint Online)
Open the SharePoint Team Site where you want to enable versioning.
Open the Shared Documents link in the Quick Launch menu.
Select the Library tab in the Library Tools ribbon.
Click on the Library Settings icon.
The Document Library Settings page is displayed. Click on the Versioning link.
The Version Settings page is displayed. Select the Create major and minor (draft) versions radio button and click on the OK button.