  The magnetic hedgehog lattice (HL), which was recently discovered in the
$B$20-type chiral magnet MnSi$_{1-x}$Ge$_x$, is a topological spin texture with
a periodic array of magnetic monopoles and anti-monopoles. Within the continuum
approximation, the monopoles and anti-monopoles are predicted to move, collide,
and pair annihilate in an applied magnetic field, but it remains unclear how
the lattice discretization affects their motions. Here, we study the
trajectories of monopoles and anti-monopoles in a lattice system by simulated
annealing with field sweep. We show that the monopoles and anti-monopoles move
and repel before pair annihilations. We also clarify that their motions are
closely related with the field dependence of the scalar spin chirality.
