Return to the Security Office after the Tunnels, follow the Dispatcher's instructions, place the Data Diver in the Inventory machine box, then leave through the newly opened garage route.
The Clock Off Ending is the most direct of Secret of the Mimic's three finales. It becomes available as soon as you return from the Tunnels with the Mimic blueprints, so it is likely the first ending you encounter.
For a different finale, use the Endings Guide to open the other two ending routes. If you need to revisit an earlier area first, use the main walkthrough to find every major game segment.
Clock Off Ending
You can trigger Clock Off immediately after leaving the Tunnels following the Tiger Rock battle. On returning to the Security Office, the Dispatcher congratulates you and says that your job is complete.
To earn the ending, follow those instructions exactly: place the Data Diver in the box on the Inventory machine to the right of the receiver. The Dispatcher then opens the Security Office door and tells you to return to the van.
Leave the Security Office and take the path opposite the door. The previously locked shutter is now open and leads to a garage containing the Fazbear Entertainment van. Enter the van to trigger the long closing cutscene.
Arnold begins driving home, but the Dispatcher soon calls in anger and says that he has not heard from you all night. The scene explains why simply leaving cannot resolve Arnold's situation. Stay through the credits, which reveal an additional detail about Arnold's fate.
After the credits, the game reloads you to the point immediately before you hand the Data Diver to the Dispatcher. From there, you can pursue one of the other endings without replaying the preceding section.