In the event dashboard, go to Registration > Registrations.
Hover your mouse on the participant and click on Quick view.
In the side panel, click Edit registration.
This will take you to the registration form where you can do the changes.
In the registration form, you can also upload a file in a File upload field, for example a contract.
Note: In Registration > Form, a File upload field shows a disabled preview. You can't upload a file there. To add a file to a registration, edit the registration.

