  Small angle neutron scattering experiments were performed on a bulk single
crystal of chiral-lattice multiferroic insulator Cu$_2$OSeO$_3$. In the absence
of an external magnetic field, helical spin order with magnetic modulation
vector $q \parallel <001>$ was identified. When a magnetic field is applied, a
triple-$q$ magnetic structure emerges normal to the field in the A-phase just
below the magnetic ordering temperature $T_c$, which suggests the formation of
a triangular lattice of skyrmions. Notably, the favorable $q$-direction in the
A-phase changes from $q \parallel <110>$ to $q \parallel <001>$ upon
approaching $T_c$. Near the phase boundary between these two states, the
external magnetic field induces a 30$^\circ$-rotation of the skyrmion lattice.
This suggests a delicate balance between the magnetic anisotropy and the spin
texture near $T_c$, such that even a small perturbation significantly affects
the ordering pattern of the skyrmions.
