HP ProOne 400 G1 All-in-One Hardware Reference Manual page 27

9. Remove the three screws securing the fire enclosure shield to the chassis, and then rotate the
shield out of the chassis.
Figure 2-15 Removing the fire enclosure shield
The memory modules can now be seen in the right side of the chassis.
Figure 2-16 Locating the memory modules
Removing and installing memory 21