If you want to quickly close down an existing job, you can do this easily by first opening the job you want to complete. From here, use the 'Edit' button (top right, beneath quick links).
At the bottom of this page, there is a section called 'Advanced options' click this tab to extend it and you will see an option to mark the job as complete. Tick this box and click 'Save'. The job is now ready for invoicing.
If you need to mark it as 'Incomplete', simply untick the checkbox and hit 'Save' once more. If a final invoice has already been raised, you will have to mark it as a 'Partial' invoice before you're actually able to mark the job as uncomplete.