You will be taken to the goal page, where you will see all information including tasks and due dates associated with it. Click complete goal.
This will prompt a new dialog box to appear which will ask you if you are sure you want to complete the goal as you will not be able to reopen it.
Tick to confirm it has been completed successfully and click complete.
When you re-load the Goals tab, you will now be able to locate the goal using the 'completed' filter.