How can I delete only selected completed tasks, not ALL comleted tasks?
Unfortunately, that's not possible directly. In order to delete one or multiple completed tasks, you'll need to uncomplete this task, then right-click on it and choose "Delete" from the context menu.
We will consider adding an option for partial deletion of completed tasks in the future. You're welcome to add a request to our Votebox: http://todoist.com/Vote/