Is it possible to populate a form from an online spreadsheet?

Is it possible to populate a form from an online spreadsheet and have the user amend his spreadsheet row entry?

I am not sure if I have understood your question correctly.

Do you mean to say that you want to populate your form fields from the data of an online spreadsheet? Unfortunately it is currently not possible to populate form field form any external sources or previous submission data. 

If you meant to send your form's submission data to google spreadsheet, it is possible to achieve your requirement by integrating your form with google spreadsheet. The following guide should help you how to integrate form with google spreadsheet:  https://www.jotform.com/help/228-How-to-Integrate-Form-with-Google-Spreadsheet 

Hope this helps.

Do get back to us if you meant to ask something else.