Click on the submission that you want to edit and hit the Edit button within the lightbox that appears. Here you can change the content provided by the end user.
After you have made the necessary changes, click on Update Submission. The entry will be changed in the Submissions section, from where you can resend it to your inbox. The CSV file containing form submissions and the default form report will take into account the new content automatically. Custom reports need to be updated (simply click the Update link) in order to reflect the new content.