If you have not yet finalized the reference request, you'll be able to modify any of the fields.
After the request has been finalized, you're no longer able to modify the fields. Nevertheless, only you and your referee will see this information, therefore you don't necessarily need to make a new request.
The only instances where you would need to make a new request is if the email address was incorrectly entered, you're requiring the reference to be specific to a different discipline than the one initially picked, or you're requiring a different reference type.
Before creating a new request, ensure you delete the request with the mistake. The DELETE button will no longer be available once a document is submitted to the request.