When your data is imported from Google Spreadsheets, you may only modify the data on Google Spreadsheet, but in the chart pop-up window.
You will notice upon successfully linking your Piktochart data with Google Spreadsheet, the data table in the Data tab has been greyed. This disables users to edit numbers in the data table.
If you would like to make any changes, please located the Google Spreadsheet that is linked to the chart in question, and modify the data on your Google Spreadsheet.
After saving your modified data, your chart will be automatically updated with new data.