To edit your file selection in the CrashPlan application first go to the home screen, then click "Details" next to the device you wish to edit.
Now click "Manage Files". If you have more than one backup set click the button for the backup set you wish to edit.
Here you are able to go through your device's file hierarchy and change what to back up. You can select entire folders, or click the folders themselves to select specific files inside a folder. If you need to go back a folder, click the relevant part of the file path above (for example, you can click "Users" to go back to that folder which is highlighted and underlined below). Remember to click "Save" at the bottom-right when you're done.
The video below demonstrates how to select files to back up using the CrashPlan Desktop App.