We present a model for the time evolution of the shape of a droplet catalyzing the vapor-liquid-solid growth of III-V semiconductor nanowires under varying group III or group V fluxes. Under the assumption of a constant radius of the nanowire top, the model gives the time dependence of the droplet contact angle. These results can be used for the fine tuning of the droplet shape, which affects the preferred crystal phase (zincblende or wurtzite) of III-V nanowires.