First export your contacts. Assuming your using Outlook or Outlook Express:
Microsoft Outlook and Outlook Express: Here are some general directions to follow, though instructions may vary by version. For more detailed instructions, open 'Help' in Outlook or Outlook Express and type 'export' in the search box. Look for topics that include 'export wizard,' 'export information,' 'exporting contacts' or 'exporting address book contacts' in the title.
Select File > Import/Export > Export from the main menu