You can view and interact with the data in PivotTables, but the PivotTable tools you’d need to make changes to the PivotTable aren’t available in Excel Online.
If you have the Excel desktop application, you can use the Open in Excel button to open the workbook and change the style of for your PivotTable there. Here’s how:
Click Open in Excel and change the style of your PivotTable.
For news about the latest Excel Online updates, visit the Microsoft Excel blog.