In Excel Web App, you can apply sorting and filtering to a PivotTable, and move its fields and change value field settings in the Field List, but you can't change its data layout.
If you have the Excel desktop application, you can use the Open in Excel button to open your workbook and change the layout there. Here’s how:
Click Open in Excel and change the data layout of a PivotTable.
For news about the latest Excel Web App updates, visit the Microsoft Excel blog.