For form owners and creators
If you've created a form, you can allow form respondents to edit their responses by checking the box labeled Allow responders to edit responses after submitting, which you can find at the bottom of your form. If you enable this option, your form respondents will be able to edit the responses they've submitted to your form. These edits will be reflected in your spreadsheet and in your summary of responses.
Form respondents will be able to change their responses by clicking Edit your response on the form submission confirmation page.
You can also edit form responses received in your spreadsheet (if, for example, you’d like to correct a typo). It is important to note, though, that if you edit a response in your spreadsheet, you will not be overriding the original response in Forms, only the copy of the response in your spreadsheet. If the original respondent edits his or her response at a later time, the edited response will override any changes made manually in the spreadsheet.Google Apps settings
If you’re a Google Apps user, you’ll see a box at the top of your forms that allows you to edit settings specific to your domain. Checking the first box, Require [your domain] login to view this form, will prevent respondents from seeing your form without first logging in. Checking the second box, Automatically collect respondent's [your domain] username, will collect respondents’ usernames and then send them an email with a copy of their form responses. That email will have an Edit your response link that will allow them to edit their form responses — that is, if you've enabled that option.
For form respondents
If the creator of a form sent to you has enabled the Allow users to edit responses option, you'll be able to edit your responses to the form. There are two ways to do this: from your email confirmation or on the form submission confirmation page.
If you check the Send me a copy of my responses box on the form, you'll receive an email confirmation showing your form responses. If you click the Edit your response link in the confirmation email, you'll be taken to a screen that allows you to edit your responses.
Once you click Submit on the form, these changes will be reflected in the form owner's spreadsheet and in the summary of responses. You can edit your responses as many times as you’d like using the Edit your response link.
You can also edit your form responses on the form submission confirmation page. On the confirmation page, click the Edit your response link. You'll be able to see your previous answers and make edits. Once you click Submit on the form, these changes will be reflected in the form owner's spreadsheet and in the summary of responses.