What's the issue you're facing?
A participant wishes to edit their response to a questionnaire. How can I allow them to do this?
Solution
Depending on your companies settings there are a couple of different options here.
Option 1
Go to the event and then go to the releant questionnaire tab. You will see all of your participants in seperate columns. At the bottom of the relevant participant there will be a drop-down allowing you to change status of submission, here you can click the 'Allow re-sub' option.

You will then see that the status for this participant will change to 'Waiting for re-submission'.
Option 2
You can allow participants to edit the answers to questionnaires after submission right up until the deadline of the event. After the deadline has passed they would not be able to re-submit in this way.
To enable this feature your Superhost or somebody with the relevant permissions for your account would need to do the following:
1. Go to the Admin tab
2. Click on Sourcing Settings
3. Ensure that the 'Allow participants to edit answers before deadline' settings is enabled
Once this change has been applied, when you participant has submitted and needs to edit answers on a questionnaire they can click on the 'Edit Answers' button up until the deadline for the event passes.