There should be a button on your Formatting Toolbar for this, next to the
"RIGHT-CENTER-LEFT" alignment buttons. Older versions of Excel did not have
this button by default, so if its not there, then you can do this:
Select your merged cell
Right-click and select "Format Cells" (or select Cells from the Format menu)
Click on the "Alignment" tab
Uncheck the box for "Merge Cells"
Click OK
HTH,
Elkar