To delete the section breaks, use the Find and Replace dialog box (Ctrl+H). Specify the following:
"Find what": ^b
"Replace with": (leave blank)
Click the Replace All button.
You can then change the number of columns for the only remaining section. Note that you may have to change other section settings which got deleted along with the section breaks; for more, see
http://word.mvps.org/faqs/formatting...thSections.htm.