Solution:

1) Open the desired mail merge main document.
2) Select 'Mail Merge...' from the 'Tools' menu. (The Mail Merge Helper dialog box appears.)
Mail Merge Helper
3) Click 'Create' from the 'Main Document' group.
4) Select 'Restore to Normal Word Document...' from the drop-down menu. (The Microsoft Word dialog box appears.)
Microsoft Word
5) Click 'Yes' to restore the main document to a normal Word document. (The Mail Merge Helper dialog box reappears.)
Mail Merge Helper
NOTE: The association between the data and/or header source and the main document is removed and the main document is converted to a normal Word document.
6) Click 'Close'. (The Microsoft Word document reappears with the mail merge toolbar removed.)
7) Select the 'File' menu and select 'Save'.