Dell OptiPlex N Service Manual page 49

4. Raise the back of the cover and pivot it toward the front of the
computer.
5. Lift the cover off the hooks at the front of the chassis.
Four plastic hooks at the front of the cover secure it to the chassis.
Before you reinstall the cover, fold all cables out of the way so that they do not
interfere with the cover or with the proper airflow inside the computer. You may
have to press gently downward on the back edge of the cover while engaging
the locking mechanism.
Removing and Replacing Parts 4-5