The exact steps will depend on how you created the list in the first place. Generally, though, you should click in the very first level 1 item of the list. (For a list which is associated with heading styles, this would mean that you click in the first Heading 1 paragraph of the document.) Then select Home > Multilevel List > Define New Multilevel List and modify each numbering level as desired. See also
How to create numbered headings or outline numbering in Word 2007 and Word 2010 | ShaunaKelly.com.