Dell OptiPlex XE3 Small Form Factor Service Manual page 58

9. Connect the following cables:
a. Intrusion switch
b. Power switch
10. Install the:
a. M.2 PCIe SSD
b. Memory module
c. Processor
d. Heat sink
e. Heat sink fan
f. Heat sink duct
g. Hard drive and optical drive module
h. HDD assembly
i. Front bezel
j. Side cover
11. Follow the procedure in
58 Демонтаж і повторний монтаж
After working inside your computer.