The Mark as Final command prevents a presentation from being edited, but it's not a security feature. Anyone who receives an electronic copy of a presentation marked as final can reverse the Mark as Final status and edit the presentation.
Turn off "Mark as Final"
To turn off Mark as Final so that you can edit a file, click Edit Anyway in the yellow message bar at the top of the PowerPoint window.
To mark a presentation as "final," see Help prevent changes to a final version of a file.
How "Mark as Final" is indicated in a file
To indicate that a presentation is marked as final, PowerPoint puts an icon in the status bar at the lower-left side of the window, as illustrated below.
When you remove the Mark as Final setting from a presentation, that icon disappears from the status bar.