SPField class

Use the Fields property of either the SPList class or the SPListItem class to return an SPFieldCollection object that represents the collection of fields for a list or list item. Use an indexer to return a single field from this collection. For example, if the collection is assigned to a variable named collFields, use collFields[index] in C#, or collFields(index) in Microsoft Visual Basic, where index is either the index number of the field in the collection or the display name of the field.

If you set properties of the SPField class and its inheriting classes, you must call the Update method for changes to take effect in the database.

The following example is a console application that gets a reference to the Expires field in the Announcements list and sets the default expiration date to one week after an item is created.

Reference