Open the task you want to delete. In the upper right corner of the task dialog, click on the ellipsis, and then select Trash from the menu that opens.
If you have deleted a task by mistake, you can still bring it back. To do this, click on the view switcher in the top bar of your project and switch from Active to Trashed.
Look for the task you have mistakenly trashed, and click on it to open the task dialog. Click on the ellipsis in the upper right corner of the dialog and then select Restore Task from the menu. The task will then be moved back to your active project board.
How to Delete Tasks for Good
There's currently no option to permanently delete tasks. There is, however, a workaround in case you really need to make a task disappear. To do this, just create a new project and move the task to this new project, then archive and subsequently delete it. Once the project itself is deleted, the task will be gone, too.