When an incident has been completed, it can be closed.
Viewing closed incidents can be done by selecting 'All closed', located in the top menu bar of the Cases dashboard.
Click on the Event ID of the incident you wish to reopen, and click the red 'Reopen' button. Please not if there are certain closure rules in place in your organisation, only those with certain access will able to reopen a case.
This case will now appear as an 'Open' case which can be edited again.