  Topological magnets exhibit fascinating properties like topologically
protected surface states or anomalous transport phenomena. While these
properties can be significantly altered by manipulating the magnetic state, the
experimental verification of such predictions remains challenging. Here, we
demonstrate the efficient magnetic field control of the Weyl semimetallic state
of the collinear ferromagnet Co$_3$Sn$_2$S$_2$ by magneto-optical spectroscopy.
We resolve a redshift of the nodal loop resonance as the magnetization is
rotated into the kagome plane by the magnetic field. Our material-specific
theory, capturing the observed field-induced spectral reconstruction, shows the
creation of 26 Weyl points for one in-plane magnetization direction and
predicts the emergence of a gapless nodal loop for the orthogonal in-plane
magnetization orientation. These findings demonstrate that while topological
band structures are generally considered robust, breaking underlying crystal
symmetries with external fields provides an efficient way to manipulate them,
even in collinear magnets. This approach opens exciting avenues to control band
topology also in materials with more complex magnetic structures and even to
study the interplay of real- and momentum-space topological states, e.g. in
skyrmion-lattice systems.
