Unmerging all merged cells

   1.   Select all cells in the worksheet.

A quick way to do so is to click the triangle at the intersection of the row headers and column headers:

   2.   On the Home tab, in the Alignment group:

• If Merge & Center isn't highlighted, there are no merged cells in the selected area.
• If the Merge & Center button is highlighted, click it:

   3.   In the Merge & Center list, select Unmerge Cells to unmerge all cells in the selected area:

See also this tip in French: Annuler la fusion de toutes les cellules fusionnées.