Simple. We know the relation v=fλv=fλ, where vv is velocity, ff is frequency and λλis wavelength.
For velocity to be constant, we get the relation f∝1λf∝1λ. This means that you can increase or decrease the frequency, and the wavelength will change appropriately so that the velocity remains constant.
Thus, only frequency/wavelength changes will not change the velocity of a wave.
