Why replace c with v in Broglie hypothesis
C is for radioactive rays while v is for the matter waves..here c is for a particle travelling with light speed, i.e., radioactive rays, so we replace it with v velocity of any particle for its matter wave

$E=mc^2$ and $E=hv$. 
Therefore,  $$mc^2=hv$$
Or $$mc^2=\frac{hc}{k}$$, where k=wavelength
$$mc=\frac{h}{k}$$
Or, $$k=\frac{h}{mc}$$
   Replacing c by v we get
$$k=\frac{h}{mv}$$
Note the difference between matter waves and electromagnetic waves. E.W. are associated with electrical and magnetic field while matter waves are not so E.w. travel at 3×10^2 m/s. While matter waves travel with different velocity. The equation of de broglie with c is valid, it can be treated for finding the wavelength of electromagnetic waves, or electrons. But the velocity of matter waves are quite smaller as compared to light speed. So he used a general v in place of c V can be equal to c when the particle is travelling at light speed