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.
Change to an option that is unavailable
When you edit a registration, an option outside its Availability period shows as Unavailable. You can't select it, even as an organizer. For example, you can't move a registrant from a free option to a paid option after the To date of the paid option.
In the event dashboard, go to Registration > Form.
Change the Availability period of the option so that it includes today. See Multiple choice options settings.
Edit the registration and select the option.