You can have up to three active course choices at one time, so you just need to submit another application for the new course. If you've already used all your choices, please email us at firstname.lastname@example.org.
To make an additional choice you'll need to withdraw one of your current three choices. Please be clear when contacting us about which existing choice you wish to withdraw. When that choice is withdrawn you'll be able to log back in to your application to add a new choice.