Intel BOXD850MVSE Quick Reference Manual page 13

3 Align the four fasteners (B) of the processor fan heatsink RM base with the
corresponding holes in the desktop board (C). Gently press the base down
until all four corners snap into place. Verify that all four fasteners are fully
engaged, then press down each of the four locking pushpins (A) to fully
secure the base to the desktop board.
A
B
C
OM12177