Huygens' metasurfaces have demonstrated almost arbitrary control over the
shape of a scattered beam, however, its spatial profile is typically fixed at
fabrication time. Dynamic reconfiguration of this beam profile with tunable
elements remains challenging, due to the need to maintain the Huygens'
condition across the tuning range. In this work, we experimentally demonstrate
that a time-varying metadevice which performs frequency conversion can steer
transmitted or reflected beams in an almost arbitrary manner, with fully
dynamic control. Our time-varying Huygens' metadevice is made of both electric
and magnetic meta-atoms with independently controlled modulation, and the phase
of this modulation is imprinted on the scattered parametric waves, controlling
their shapes and directions. We develop a theory which shows how the scattering
directionality, phase and conversion efficiency of sidebands can be manipulated
almost arbitrarily. We demonstrate novel effects including all-angle beam
steering and frequency-multiplexed functionalities at microwave frequencies
around 4 GHz, using varactor diodes as tunable elements. We believe that the
concept can be extended to other frequency bands, enabling metasurfaces with
arbitrary phase pattern that can be dynamically tuned over the complete 2\pi
range