1. Click setup
2. Under App Setup on the left side-bar, expand customize (Note: for custom objects, expand create instead and click Objects to view a list)
3. Expand the standard object you would like to edit the standard field for Click Fields
4. If the field can be edited, in the standard field list, there will be an edit button on the left
5. Click edit
You can customize several aspects of standard fields, such as the values in picklists, the format for auto-number fields, tracking field history, lookup filters on relationship fields, and field-level help.