Lenovo ThinkEdge SE455 V3 User Manual page 222

Step 3. Pull the plunger up; then, hold the plunger and the handle, and slide the system board slightly
backward to disengage it from the chassis.
Figure 191. Disengaging the system board
Step 4. Rotate the left end of the system board up to remove the system board from the chassis.
214 ThinkEdge SE455 V3 User Guide