//Get the merged cells list to put it into the arraylist object       
ArrayList<CellArea> al = worksheet.getCells().getMergedCells();

//Define cellarea
CellArea ca;

//Define some variables
int frow, fcol, erow, ecol;

// Print Message
System.out.println("Merged Areas: \n"+ al.toString());

//Loop through the arraylist and get each cellarea to unmerge it
for(int i = al.size()-1 ; i > -1; i--)
{ 
    ca = new CellArea();
    ca = (CellArea)al.get(i);
    frow = ca.StartRow;
    fcol = ca.StartColumn;
    erow = ca.EndRow;
    ecol = ca.EndColumn;
    System.out.println((i+1) + ". [" + fcol +"," + frow +"] " + "[" + ecol +"," + erow +"]");
    worksheet.getCells().unMerge(frow, fcol, erow, ecol);
}