Yes, you may delete or add a referee until your application has been submitted.
In your application form, we invite you to go into the academic background section and click on the check box which appears on the right of the referee from whom a reference is pending. A pop up box will appear with the question :"Are you sure you want to suppress this reference?" If yes, please accept. You will then be able to add a new referee
Make sure you inform in advance your referee of your request as an automatic notification email will be sent once you've filled out their data.