Lenovo ThinkCentre M90n-1 Hardware Maintenance Manual page 74

Avoid contact with the hot computer. During operation, the computer becomes hot enough to burn the skin.
Before you touch the computer, turn off the computer, disconnect power, and wait approximately 30 minutes
for the computer to cool.
Replacement procedure
1. Remove any media from the drives and turn off all connected devices and the computer.
2. Disconnect the power cord from the electrical outlet and disconnect all cables from the computer.
3. Unlock any locking device that secures the computer cover.
4. Remove the bracket, if any. See "DIN rail bracket kit" on page 58, "VESA mount bracket" on page 60,
and "Power adapter bracket" on page 62.
5. Turn over the computer so that the bottom cover is facing up.
6. Remove the bottom cover.
7. Install the bottom cover.
8. Turn over the computer so that the bottom cover is facing down.
9. Reinstall all removed parts.
10. Reconnect the external cables and power adapter to the corresponding connectors on the computer,
and then reconnect the power cord to the electrical outlet.
Note: If a locking device is available, use it to lock the computer.
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M90n-1 Hardware Maintenance Manual