Lenovo 300 Maintenance Manual page 42

Step 10. Lift up the middle cover as shown to remove it.
Step 11. To reattach the rear cover:
a. Line up the rear cover with chassis, then place the rear cover back.
b. Secure the rear cover to the chassis with the six screws.
Step 12. Reattach the hinge, foot cover and stand base.
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Lenovo All-In-One 300 Computer Hardware Maintenance Manual