All PivotTables you can view and interact with in an Excel Online workbook were created in the Excel desktop application. Therefore, you won’t be able to change their source data.
If you have the Excel desktop application, you can use the Open in Excel button to open the workbook and change the source data for your PivotTable there. Here’s how:
Click Open in Excel and change the source data for your PivotTable.
For news about the latest Excel Online updates, visit the Microsoft Excel blog.