Note: Make sure you are logged in and in the correct event
If you would like to make changes to a registered participant, use this article. Note however that they will not automatically receive a notification from this. Another article explains how to do this if you would like that.
Go to participants and view participant listing in 'registration status. You can also use the quick search bar if you look for a specific participant.
Click on the "Actions" tab on the participant's listing/segment and then select "Edit registration".
You can manually edit any of the desired registration fields. Don't forget to save!
Note: You cannot change anything in the products section, since these have to be bought. If you want to change anything in products you need to do update the registration via the participant's view in an incognito window.