In Microsoft Word 2003

1. Open the existing mail merge letter in Microsoft Word
2. Select Tools, Options from the menu bar
3. Select the View tab
4. Mark the Field Codes checkbox
5. Click OK
6. Make any necessary edits
7. Save the document

In Microsoft Word 2007

1. Open the existing mail merge letter in Microsoft Word
2. Click the Microsoft Office Button, select Word Options, and click on Advanced
3. Under the heading Show Document Content, check the box that says Show field codes instead of their values
4. Make any necessary edits
5. Save the document


In Microsoft Word 2010

1. Open the existing mail merge letter in Microsoft Word
2. Go to the File tab, select Options, and click on Advanced
3. Under the heading Show Document Content, check the box that says Show field codes instead of their values
4. Make any necessary edits
5. Save the document