If you have saved the Edit Response link, then you can only update. In case, you have not saved it, just resubmit and follow the below process.
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Spreadsheet update/entry process
1. please give your input through http://tinyurl.com/FSW14-Entry
2. After successful entry into the form, you will be redirected to a page. Please click on the "Edit your response"
3. Now you will redirected to another page, where you will find all your current entry in a from. For change or add entry in future either bookmark the page or copy and save the address from the address bar.
Thanks and regards.