Select the backup set you wish to modify and navigate to "source" down the left hand side. From here you can either edit the selected tick boxes for Desktop, Documents Favorites etc, or click on "I would like to choose the files to backup" to get a more in-depth look at what you are backing up.

Selecting "I would like to choose the files to backup" gives you a similar view to the below, from here you will be able to expand folders to see other folders contained so you can select what you would like to backup.  If you want to be selective on a file level basis, I would recommend selecting "show files"

The grey square next to a file indicated there is an item inside this location that is being backed up. If that square has a tick in, that whole item (be it folder or file) is selected for backup.

When you select "show files" you view changes and will be similar to the below. This will show you on the right the available folders and files for the location selected on the left. From here you can deselect or select items as you wish.

Once completed press "ok" on the bottom right, and then "save" on the source window. If you want to you can run a manual backup so you have a copy of these new changes, or you can wait for the next scheduled backup to run.