From your profile page, click on Pitches.

Here you can see all of the projects that you've pitched your music too. To view the tracks you've submitted, click on the "Show Tracks" button.

To change which tracks are submitted to the project, click on "Update Tracks".

From here you can de-select/select the tracks you wish to pitch. 

Make your changes, and then press "Ok" when you're done.

Done! You have now updated the tracks that are pitched to the project.