If you have started applying for one certain course without submitting, you could log in your CUCAS account, find the application in "My Application", then you will see all the actions that you can do on the right column of your application.

If you have paid for the application fee, the application status would be changed to "submitted". You cannot edit this application by that time unless the status being changed to "returned", which means you missed some personal information that university required. You should carefully read the school's response and edit your application as request.