unmerge all cells a workbook from another workbook

[jolivanes]

Is the workbook where the cells need to be unmerged open?

If it is closed, you could try this. (Change the path and name references as required)

Sub Maybe()
    Dim wb2 As Workbook
    Application.ScreenUpdating = False
    Workbooks.Open Filename:="C:\Some Folder\TempBook.xlsm"
    Set wb2 = ActiveWorkbook
    wb2.Sheets(1).Cells.UnMerge
    wb2.Close True
    Application.ScreenUpdating = True
End Sub