You can edit a Task at any time for any number of reasons. One primary reason is to "close" the Task or "mark it as complete" once the work has been done.
The easiest way to do this is to select the Task in the Project Scheduling Editor and click the Percent Complete dropdown in the ribbon:
When a Task is marked as 100% complete, the Items in the Task will automatically be marked as "Installed" and the "Install Date" will populate:
This automatic marking of "Installed" and filling the "Installed" date can be turned off via a Project Setting:
After marking a Task as 100% complete, you may want to fill in the actual hours for your Resources assigned to the Task. You can open the Task for edit and fill in the Actual Hours on the Resources tab:
You can then run comparison reports for actual vs. estimated hours. See Scheduling Reports.