Use the Edit > Replace dialog. Click the More button to expand the
bottom half of the dialog. With the cursor in the Find What box, click
the Special button, and choose "Section break" near the bottom of the
list. This puts the code ^b in the Find What box. Leave the Replace
With box empty. Click the Replace All button.

Now remove the ^b from the Find What box. Click the Special button and
choose "Column break" from the middle of the list. This puts the code
^n in the Find What box. Click the Replace All button.

--
Regards,
Jay Freedman
Microsoft Word MVP        FAQ: http://word.mvps.org
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Use Find and Replace. ^m matches page breaks; ^n matches column breaks.
Replace with nothing.