  Achieving control over magnon spin currents in insulating magnets - where
dissipation due to Joule heating is highly suppressed - is an active area of
research that could lead to energy-efficient spintronics applications. However,
magnon spin currents supported by conventional systems with uniform magnetic
order have proven hard to control. An alternative approach that relies on
topologically protected magnonic edge states of spatially periodic magnetic
textures has recently emerged. A prime example of such textures is the
ferromagnetic skyrmion crystal which hosts chiral edge states providing a
platform for magnon spin currents. Here, we show, for the first time, an
external magnetic field can drive a topological phase transition in the spin
wave spectrum of a ferromagnetic skyrmion crystal. The topological phase
transition is signaled by the closing of a low-energy bulk magnon gap at a
critical field. In the topological phase, below the critical field, two
topologically protected chiral magnonic edge states lie within this gap, but
they unravel in the trivial phase, above the critical field. Remarkably, the
topological phase transition involves an inversion of two magnon bands that at
the $\Gamma$ point correspond to the breathing and anticlockwise modes of the
skyrmions in the crystal. Our findings suggest that an external magnetic field
could be used as a knob to switch on and off magnon spin currents carried by
topologically protected chiral magnonic edge states.
