If you're using a table... you'd need to merge the top two cells so that row
1 has just one cell. Select the two cells, right-click the selection, and
choose Merge Cells.

If this is actual column formatting, you'll need to insert a continuous
section break where you want the centered topic/title to appear (Page Layout
tab, Breaks, Continuous), then set the formatting of the new section to 1
column.

One column: centered title

-------continuous section break-----------

Two columns

As Herb says, you need a Continuous section break (or two). You can make
Word insert these for you by selecting the text in question and changing the
number of columns. See http://word.mvps.org/FAQs/Formatting/UsingColumns.htm
for tips on working with columns.