If the velocity of electron increases, its specific charge

$\text{Mass\hspace{0.17em}of\hspace{0.17em}electron}=\frac{\text{rest\hspace{0.17em}mass\hspace{0.17em}of\hspace{0.17em}electron}}{\sqrt{\text{1}-\frac{{\text{V}}^{\text{2}}}{{\text{C}}^{\text{2}}}}}$
V = Velocity of electron 
C = Velocity of light
As velocity of electron increases,$\frac{{\text{V}}^{\text{2}}}{{\text{C}}^{\text{2}}}$   increases and $\left(\text{1}-\frac{{\text{V}}^{\text{2}}}{{\text{C}}^{\text{2}}}\right)$  decreases as denominator decreases mass of electron increases (rest mass electron is a constant) and thus  e/m (specific charge) decreases